Pressure Augmented Mouse

ABSTRACT

The use of a uni-pressure and dual-pressure augmented mouse permits users to simultaneously control cursor positions as well as multiple levels of discrete action modes for common desktop application tasks. One, two or more independent pressure sensors can be mounted onto several locations on the body of the mouse. Various selection techniques are described to control many discrete levels and to simultaneously control different variable functions with pressure sensors on an input device for an electronic device.

FIELD OF THE INVENTION

The present invention relates to an input device for an electronicdevice, for example a computer, and more particularly relates to acomputer input device having one or more pressure sensitive switchesarranged to generate a range of pressure values corresponding todifferent pressures being applied by a user.

BACKGROUND

What seems to be a natural addition to the next generation of mice isapparent in Apple's MightyMouse™ [10] in which two pressure buttons areavailable on each side of the mouse. Although, pressure based input isfeatured in many digitizers and TabletPCs and has been widely studied[3,11,15,16], little is known about the limitations to pressure basedinput using a mouse. This lack of knowledge may explain why the pressurebuttons available on the MightyMouse™ do not supply continuous pressurevalues and operate similar to a two-state button. One possible reason isthat there is not sufficient knowledge on the limitations and benefitsof a pressure augmented input with a mouse.

Designers can naively augment a mouse by adding a pressure sensor to afixed location on the mouse. This approach, while providing anadditional input dimension to most mouse-based interactions, can also belimiting. The location of the sensor may not be appropriate forinteracting with some of the major features of a mouse, such asclicking. Additionally, a poorly augmented mouse would restrict users toa limited number of pressure levels [11,15]. Furthermore, selectionmechanisms would be limited to the current methods for selectingpressure values, such as quick release or dwell [15]. Finally, a simpleaugmentation may not facilitate bi-directional pressure input (wherepressure control starts at 0 and moves to a higher pressure and thereverse).

Understanding the limitations and benefits of pressure based input witha mouse can allow designers to augment the mouse with pressure sensors(FIG. 1) and utilize the augmented device in a variety of novelcontexts. To effectively harness the potential of a pressure augmentedmouse designers need to know where to mount the pressure sensors on themouse, create some mechanisms for controlling pressure input, andidentify methods for selecting a pressure value.

RELATED LITERATURE

We review the literature in two related areas: augmented mice andpressure-based interaction.

Augmented Mice

The traditional two-button mouse has been augmented in numerous wayssuch as by adding multiple buttons, by providing tactile feedback or byserving as a device with more than two degrees-of-freedom.

Manufacturers continue to add buttons to the mouse. Multiple secondarybuttons make certain tasks easier but they require that users rememberthe mappings between the button and its function and in some casesrequire repositioning the fingers to facilitate input. The scroll wheelis a variation of a button that allows users to scroll vertically orhorizontally in a window. Studies show that the scroll wheel isparticularly useful in navigating through long documents [5,20].

The tactile mouse [1] contains a small actuator that makes the mousevibrate under certain conditions. This form of feedback can inform theuser when the cursor is moving into different areas of a window or whenthe user is crossing window boundaries. Akamatstu et al. [1] conducted astudy to compare the effect of tactile feedback in a mouse with visualand auditory feedback. Their results show that users complete selectiontasks better with tactile feedback over visual and auditory conditions[1].

The Rockin' Mouse [2] augments the mouse with tilt sensors. The Rockin'Mouse has a rounded bottom which allows users to tilt it and controlobjects in 3D. Balakrishnan et al. [21 show that in a 3D objectpositioning task users were 30% with the Rockin' Mouse over theconventional mouse.

The VideoMouse (61 augmented the mouse by adding a video camera as itsinput sensor. In the VideoMouse a real-time vision algorithm determinesthe six degree-of-freedom mouse position, which consists of x-y motion,tilts in the forward/backward and left/right axes, rotation of the mousearound the z-axis and limited height sensing. As a result the VideoMousefacilitates a number of 3D manipulation tasks.

MacKenzie et al. [9] designed a two-ball mouse by adding an additionalball to capture angular movement along the z-axis. The angular motion iscomputed based on simple calculations on the two sets of x-ydisplacement data. This enhancement makes rotation tasks easier toperform.

Siio et al. [17] introduced the FieldMouse which augments the mouse withan ID recognizer similar to a barcode reader. With the FieldMouse, userscan interact with virtual objects using any flat surface that isembedded with ID recognizers, such as a paper book.

Pressure Based Interaction

Numerous studies have proposed novel interaction techniques orinvestigated different applications and offered guidelines for workingwith pressure based input.

Ramos et al. [15] explored the design space of pressure basedinteraction with styluses. They proposed a set of pressure widgets thatoperate based on the users' ability to effectively control a discreteset of pressure values. Ramos et al. [15] identified that adequatecontrol of pressure values is tightly coupled to a fixed number ofdiscrete pressure levels (six maximum levels), the type of selectionmechanism and a high degree of visual feedback, However, their resultsare mainly applicable to the use of pressure based input on a stylus andthey did not examine the design space resulting from more than onepressure sensor.

Mizobuchi et al. (11] conducted a study to investigate how accuratelypeople control pressure exerted on a pen-based device. Their resultsshow that continuous visual feedback is better than discrete visualfeedback, users can better control forces that are smaller than 3N, and5 to 7 levels of pressure are appropriate for accurate discriminationand control of input values. Their results apply to pen based pressureand they do not investigate multi-pressure input.

Isometric input devices are common and use pressure based input tocontrol the mouse cursor speed. The pointing stick is a pressuresensitive nub used like, a joystick on laptops. Users decrease orincrease the amount of force on the nub to control the velocity of themouse cursor. Similarly, the PalmMouse™ [12] allows users to controlcursor speed by applying a slight amount of pressure to a navigationdome which is placed on the top of the mouse. Both examples map pressureinput to the speed of the cursor.

Researchers studied pressure input in the context of multilevelinteraction. Zeleznik et al. [19] proposed an additional “pop-through”state to the mechanical operation of the mouse button. As a result, anumber of techniques can take advantage of a soft-press and a hard-presson a pop-through button. Forlines et al. [4] proposed an intermediary“glimpse” state to facilitate various editing tasks. With glimpse userscan preview the effects of their editing without executing any commands.Multi-level input can facilitate navigation, editing or selection tasksbut utilize pressure input in a limited way.

Touch-pads that sense pressure are widespread input devices in notebooksor portable music players. Blasko and Feiner [3] proposed multiplepressure-sensitive strips by segmenting a touchpad into differentregions. They show that pressure-sensitive strips do not require visualfeedback and users can control a large number of widgets using theirfingers. Rekimoto and Schwesig [16] propose a touchpad based pressuresensing device called PreSensell that recognizes position, contact areaand pressure of a user's finger. PreSensell eliminates the need forvisual feedback by providing tactile feedback on the amount of pressurebeing applied. Unlike many of the previously discussed pressure basedmechanisms, PreSensell allows users to control bi-directional pressureinput (i.e. from 0 to the highest pressure level as well as thereverse).

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided an inputdevice for an electronic device comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising:

-   -   a first switch arranged to generate a first control signal when        depressed by the user;    -   a second switch arranged to generate a second control signal        when depressed by the user;    -   at least the second switch comprising a pressure sensitive        switch arranged to generate continuous pressure values in at        least two different identifiable discrete pressure ranges        corresponding to different pressures being applied by the user        in depressing the pressure sensitive switch.

When the input device is used with a computer including a plurality ofsequential selection items to be selected in a range from a firstselection item to a last selection item, each discrete pressure range ofthe pressure sensitive switch preferably corresponds to one of theselection items. Accordingly increasing pressure applied to the pressuresensitive switch advances the selection item being selected towards thelast selection item and reducing pressure applied to the pressuresensitive switch returns the selection item being selected towards thefirst selection item.

In some embodiments, the first switch accompanying the pressuresensitive switch comprises a two state button which is not sensitive todifferent applied pressures.

The first control signal generated by the first switch may be arrangedto confirm entry to the computer of a selected one of the discretepressure ranges of the second switch. Other method of confirming entryto the computer of the selected one of the discrete pressure rangesinclude dwelling in the pressure range or rapidly removing pressure fromthe pressure sensitive switch.

The combination of switches described herein is well suited for use witha computer mouse comprising a housing which is generally arranged to bereceived in a palm of a hand of a user and electronic circuitry whichincludes a tracking mechanism arranged to generate control signals inresponse to relative movement between the housing and a supportingsurface receiving the housing thereon. In this instance, the switchesare preferably arranged to be readily accessible by fingers of the userwhen the housing is received in the palm of the hand of the user.

When the first switch accompanying the pressure sensitive switch is atwo state switch, it is preferably arranged to be actuated by an indexfinger of the user, while the pressure sensitive switch is arranged tobe actuated by either a middle finger or a thumb of the user.

When using the input device with a computer comprising a plurality ofsequential selection items arranged in a plurality of groups, thepressure sensitive switch is preferably arranged to generate both:

i) an advancing control signal arranged to advance selection of onegroup through the plurality of groups when the pressure sensitive switchis momentarily depressed; and

ii) control signals in the form of continuous pressure values whereinthe discrete pressure ranges are arranged to correspond to the selectionitems of the selected group when continuous pressure is applied to thepressure sensitive switch for selecting one of the selection itemswithin the selected group.

When the electronic circuitry comprises two pressure sensitive switches,both are preferably arranged similarly to one another but the advancingcontrol signals are preferably in opposing directions relative to oneanother when the switches are momentarily depressed.

In some embodiments, both the first switch and the second switchcomprise a pressure sensitive switch arranged to generate continuouspressure values in at least two different identifiable discrete pressureranges corresponding to different pressures being applied by the user indepressing the pressure sensitive switch.

The two pressure sensitive switches may also be combined with a computermouse including a tracking mechanism to track relative movement of themouse and one or more two-state switches for confirming entry ofselections to the computer. In this instance, one of the pressuresensitive switches is preferably arranged to be readily accessible by amiddle finger of the user, and the other one of the pressure sensitiveswitches is preferably arranged to be readily accessible by a thumb ofthe user in the normal operating position of the mouse.

When used with a computer comprising a plurality of sequential selectionitems arranged in a plurality of groups, the first switch is preferablyarranged to generate an advancing control signal arranged to advanceselection of one selected group through the plurality of groups when thefirst switch is momentarily depressed. In this instance, the discretepressure ranges of the pressure sensitive switch are preferably arrangedto correspond to the selection items of the selected group whencontinuous pressure is applied to the pressure sensitive switch forselecting one of the selection items within the selected group.

When used with a computer comprising a plurality of sequential selectionitems arranged in cascading levels, the discrete pressure ranges of thepressure sensitive switches are preferably arranged to correspond to theselection items of alternating cascading levels. Within each level,continuous pressure may be applied to the pressure sensitive switch forselecting one of the selection items within a selected level. Switchingapplied pressure between the two pressure switches is preferablyarranged to generate a control signal which confirms entry to thecomputer of the selected item within each level to proceed to selectionof items within the next cascading level.

According to a second aspect of the present invention there is providedan input device for an electronic device comprising a pressure sensitiveswitch arranged to generate continuous pressure values over a range ofpressure values to be transmitted to the electronic device, theimprovement comprising:

a discretization function arranged to divide the range of pressurevalues into discrete pressures units according to the followingequation:

$y = \left\{ \begin{matrix}\begin{matrix}{{{floor}\left( \frac{\left( {x - r} \right)*\left( {l - 1} \right)}{R - r} \right)} + 1} \\0\end{matrix} & \begin{matrix}{x > {r - \frac{R - r}{l - 1}}} \\{x \leq {r - \frac{R - r}{l - 1}}}\end{matrix}\end{matrix} \right.$

where x is the raw pressure value from the pressure switch, I is thenumber of pressure ranges, r is the fisheye radius, and R is the totalnumber of raw pressure values.

According to a further aspect of the present invention there is providedan input device for an electronic device including a plurality ofsequential selection items to be selected in a range from a firstselection item to a last selection item, the input device comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising a pressure switch arranged togenerate a control signal comprising continuous pressure values in atleast two different identifiable discrete pressure ranges correspondingto different pressures being applied by the user in depressing thepressure sensitive switch; each discrete pressure range of the pressuresensitive switch corresponding to one of the selection items;

the electronic circuitry being arranged such that increasing pressureapplied to the pressure sensitive switch advances the selection itembeing selected towards the last selection item and reducing pressureapplied to the pressure sensitive switch returns the selection itembeing selected towards the first selection item.

According to a further aspect of the present invention there is providedan input device for an electronic device comprising a plurality ofsequential selection items arranged in a plurality of groups, the inputdevice comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising a pressure switch arranged togenerate a control signal when depressed by the user comprisingcontinuous pressure values in at least two different identifiablediscrete pressure ranges corresponding to different pressures beingapplied by the user in depressing the pressure sensitive switch;

wherein the pressure sensitive switch is arranged to generate anadvancing control signal arranged to advance selection of one groupthrough the plurality of groups when the pressure sensitive switch ismomentarily depressed and wherein the discrete pressure ranges arearranged to correspond to the selection items of the selected group whencontinuous pressure is applied to the pressure sensitive switch forselecting one of the selection items within the selected group.

When the electronic circuitry comprises two pressure sensitive switches,the pressure sensitive switches are preferably arranged to generateadvancing control signals arranged to advance selection of one groupthrough the plurality of groups in opposing directions relative to oneanother when the pressure sensitive switches are momentarily depressedand wherein the discrete pressure ranges of each pressure sensitiveswitch are arranged to correspond to the selection items of the selectedgroup when continuous pressure is applied to the pressure sensitiveswitch for selecting one of the selection items within the selectedgroup.

According to a further aspect of the present invention there is providedan input device for an electronic device comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising a pressure switch arranged togenerate a control signal when depressed by the user comprisingcontinuous pressure values in at least two different identifiablediscrete pressure ranges corresponding to different pressures beingapplied by the user in depressing the pressure sensitive switch;

the pressure switch being operable in a first mode in which a variablefunction associated with the pressure switch is arranged to be varied ina first direction responsive to increased pressure applied to the switchand a second mode in which the variable function is arranged to bevaried in a second direction opposite to the first direction responsiveto increased pressure applied to the switch.

There may be provided an auxiliary switch arranged to convert thepressure switch between the first and second modes upon activation ofthe auxiliary switch.

The variable function may be arranged to be varied by the pressureswitch responsive to a continuous pressure being applied to the pressureswitch. The pressure switch may be further arranged to be convertedbetween the first and second modes responsive to a momentary pressureapplied to the pressure switch.

According to a further aspect of the present invention there is providedan input device for an electronic device comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising:

-   -   a pressure switch arranged to generate a control signal when        depressed by the user comprising continuous pressure values in        at least two different identifiable discrete pressure ranges        corresponding to different pressures being applied by the user        in depressing the pressure sensitive switch; and    -   a tracking mechanism arranged to track movement of the housing        relative to a supporting surface;    -   the pressure switch and the tracking mechanism being arranged to        respectively controllably vary two different variable functions        simultaneously.

The two different variable functions may comprise manipulations of anobject, for example a translation of an object, a rotation of an objectand/or a zoom function.

Preferably the pressure switch is arranged to generate pressure valuesin approximately five to ten different identifiable discrete pressureranges.

An increase in pressure applied to the pressure switch may be arrangedto controllably vary one of the variable functions in one direction anda decrease in pressure applied to the pressure switch may be arranged tocontrollably vary said one of the variable functions in an opposingsecond direction.

There may be provided an auxiliary switch arranged to fix one of thevariable functions associated with the pressure switch upon activationof the auxiliary switch.

When there is provided two pressure switches associated with one of thevariable functions, the two switches may be arranged such that anincrease in pressure to one of the pressure switches controllably variesthe variable function in one direction and an increase in pressureapplied to the other pressure switch controllably varies the variablefunction in the opposing direction.

When the pressure switch is arranged to controllably vary one of thevariable functions in one direction, an increase in pressure to thepressure switch may be arranged to correspond to an increase in a rateof variation of the variable function.

The pressure switch may be arranged to controllably vary the variablefunction through a range of values when continuous pressure is appliedand may be arranged to vary the variable function in prescribedincrements when momentarily depressed.

There may be provided an auxiliary switch arranged to convert thepressure switch between a coarse mode and a fine mode, wherein in eachmode the pressure switch is arranged to controllably vary one of thevariable functions in increments according to the discrete pressureranges applied by the user with the increments in the coarse mode beinggreater than the increments in the fine mode.

Alternatively, in the coarse mode the pressure switch may be arranged tocontrollably vary the variable function associated therewith accordingto variation in pressure applied by the user, and the fine mode thepressure switch may be arranged to controllably vary the variablefunction associated therewith according to different pressures appliedby the user at a slower rate than the coarse mode.

According to another aspect of the present invention there is providedan input device for an electronic device comprising a selection functionand an action initiation function, the input device comprising:

a housing;

electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device;

the electronic circuitry comprising a pressure switch arranged togenerate continuous pressure values in at least two differentidentifiable discrete pressure ranges corresponding to differentpressures being applied by the user in depressing the pressure sensitiveswitch;

the pressure switch being arranged to generate a first signal responsiveto a first user interaction and a second signal responsive to a seconduser interaction, the pressure switch being arranged to generate aselection signal responsive to the first and second signals beinggenerated in which the selection signal is identifiable as a selectionby the selection function of the electronic device.

The pressure switch may be arranged to generate the first signalresponsive to applying a pressure to the pressure switch which exceeds afirst pressure threshold and to generate the second signal responsive toapplied pressure to the pressure switch falling below a second pressurethreshold.

The pressure switch may be arranged to generate an audio signal with thesecond signal.

Alternatively, the pressure switch may be arranged to generate thesecond signal responsive to a pressure being released from the pressureswitch within a prescribed duration from the first signal. In thisinstance, the pressure switch may be arranged to generate an audiosignal if pressure is not released from the pressure switch within theprescribed duration from the first signal.

The pressure switch may be arranged to generate an action initiationsignal responsive to two selection signals being generated within aprescribed period of time in which the action initiation signal isidentifiable as an initiation of an action by the action initiationfunction of the electronic device. In this instance, the first signalmay correspond to a pressure applied to the pressure switch whichexceeds a first pressure threshold and the second signal corresponds toan applied pressure falling below a second pressure threshold.Alternatively, the pressure switch may be arranged to generate thesecond signal responsive to pressure being released from the pressureswitch within a prescribed duration from the first signal beinggenerated.

The pressure switch may also be arranged to generate an actioninitiation signal identifiable as an initiation of an action by theaction initiation function of the electronic device responsive to apressure being applied to the pressure switch which exceeds an upperpressure threshold which is greater than any pressure thresholdsassociated with the first and second signals. Preferably the upperthreshold is arranged to be adjusted by a user.

Some embodiments of the invention will now be described in conjunctionwith the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a computer mouse augmented with two pressure sensors.

FIG. 2 is an illustration of different discretization functionsincluding: (a) DF1: Linear, (b) DF2: Quadratic centered at the lowerrange, (c) DF3: Quadratic centered in the middle range.

FIG. 3 illustrates the location of targets in one of four differentrelative pressure distances based on the pressure level.

FIG. 4 illustrates the Mean Completion times for each (a) selectiontechnique [left] and (b) sensor location [right].

FIG. 5 illustrates the Average Crossings for (a) each technique [left]and (b) sensor location [right].

FIG. 6 illustrates the Categorization of pressure levels in terms ofcoarse-level and fine-level items.

FIG. 7 illustrates Mean completion times for each control mechanism.

FIG. 8 illustrates Mean crossings for each control mechanism.

FIG. 9 illustrates a trace of applied pressure over time of a typicaluser control when using the top sensor with the click-technique forpressure levels (a) 8 and (b) 10 when selecting a target at a distanceof 815 pressure pixels.

FIG. 10 is an illustration of the details of a fisheye discretizationfunction. The purpose of the fisheye function is to allow for smootherand more accurate control of the pressure selection mechanisms. The sizeof each pressure level is adjusted according to the current position ofthe pressure cursor. Larger space is reserved for the current pressurelevel. The amount of pressure units reserved for the fisheye is definedby the radius, r. The figure also presents the relationship between allthe elements involved in the fisheye function.

FIG. 11 (a) is a schematic illustration of a pressure menu according toa Fisheye discretization.

FIG. 11 (b) illustrates a computer mouse augmented with one pressuresensor.

FIG. 12 illustrates a target selection with a cursor.

FIG. 13 illustrates average performance of the different functions fromleft to right with performance measures of (a) Movement Time (MT); (b)Errors (E); and (c) Number of Crossings (C).

FIGS. 14 (a) and (b) graphically illustrate average performance of FEand L across various pressure levels for Movement time and Error ratesrespectively.

FIG. 15 (a) illustrates a computer mouse augmented with two pressuresensors.

FIG. 15 (b) schematically illustrates rotation of a triangular objectwith pressure input and simultaneous displacement using mouse movementas an exemplary task which is common in several applications.

FIG. 16 illustrates a cursor state in various PressureMove techniquesincluding: (a) a standard cursor without any pressure applied to it; (b)a cursor filling up when pressure is being applied; (c) movement in aclockwise direction; (d) movement in a counter-clockwise direction; and(e) a hierarchical manner, for first pressure level, wherein the arrowsare not part of the cursor and only used to indicate how the cursormoves.

FIG. 17 illustrates the pressure mapping functions for each of thePressureMove techniques comprising: (a) Naive implementation; (b)Rate-based technique; (c) Hierarchical technique; and (d) Hybridtechnique. The dotted horizontal line in FIGS. 17 (c) and (d) atAngle=150 indicates a left-click action.

FIG. 18 is schematic representation of an experimental task consistingof docking a triangular shaped object over a target in which rotationcan be controlled using pressure, and displacement can be controlledwith mouse movement.

FIG. 19 is a graphic representation of a mean trial completion time(along the Y-axis in seconds) with standard error bars (a) for eachtechnique and (b) for each technique and target-fit.

FIG. 20 is a graphic representation of (a) Mean scores for differenttechniques and (b) Median user-ranking of different techniques in termsof Mental Demand, Overall Effort and Performance.

FIG. 21 is a graphic representation of comparison of the techniques overeach block for (a) MT; and (b) Crossing.

FIG. 22 is a graphic representation of (a) Mean MT for each techniqueand orientation; and (b) Mean number of clicks of Hierarchical andHybrid techniques for each orientation.

FIG. 23 is a graphic representation of traces of a typical user controlwhen using the four PressureMove techniques in which the patterns revealthe degree of simultaneity employed in each of the techniques, rangingfrom low simultaneity with the Naive technique to high simultaneity withRate-based.

FIG. 24 (a) illustrates a computer mouse augmented with a pressuresensor on top of the mouse button.

FIG. 24 (b) graphically illustrates a pressure sensor for activating amouse-down and mouse-up events typical of a mouse click.

FIG. 25 is a graphic representation of a pressure click in which a mousedown is invoked after providing a pressure of 4 units and a mouse up isrecorded when releasing immediately after a mouse down and when thepressure level is equal of less than 2 units.

FIG. 26 is a graphic representation of use of a pressure tap to triggera click when the user presses up to a threshold and releases within 150ms wherein if the release takes place after 150 ms then the system doesnot record a click.

FIG. 27 is a graphic representation of a HardPress which triggers adouble-click when the user presses beyond a threshold which issignificantly higher than that required to do a single pressure clickand for a Pressure Click in which a double-click is invoked by twoconsecutive pressure clicks.

FIG. 28 (a) illustrates mean completion time with standard error.

FIG. 28 (b) illustrates right: total errors for each interaction modeincluding: BC—Button Click; PC—Pressure-Click; PC+A—Pressure click withaudio; PT—Pressure-Tap; and PT+A—Pressure Tap with audio.

FIG. 29 is a graphic representation of a Time vs. Pressure plot fortypical single click actions with Pressure Tap and Pressure Click.

FIG. 30 is a graphic representation of mean completion time: (a) withstandard error; and (b) with total errors for each interaction modes.

FIG. 31 is a graphic representation of: (a) an overall user ranking ofdouble-click techniques, and (b) double-click timeouts for eachinteraction mode.

FIG. 32 is a representation of (a) state transitions for commonoperations with a mouse; and (b) a variation of Buxton's three-statemodel for facilitating the task of dragging with PButtons.

DETAILED DESCRIPTION

Referring to the accompanying figures there is illustrated an inputdevice generally indicated by reference numeral 10. The device 10 inillustrated embodiment comprises a mouse for a computer including ahousing 12 arranged to be received in the palm of a user's hand. Inother embodiments, the device 10 may assume other forms while stilltaking advantage of many features of the present invention as definedfurther herein. For instance, the input device 10 may be used for anytype of electronic device requiring an input, for example, a computer, acellular phone, video games, personal electronic assistants, and thelike.

The housing 12 of the illustrated mouse includes electronic circuitrytherein which is arranged to detect user inputs and to generaterespective control signals corresponding to the user inputs which are inturn transmitted to the computer for controlling operation of thecomputer. As in a typical computer mouse, the device also includes inpreferred embodiments a tracking mechanism arranged to track movement ofthe mouse housing relative to a supporting surface, for example a table,supporting the mouse thereon. In typical applications, the computermouse includes a left click switch 14 and a right click switch 16 eachcomprising two state buttons operable between an inactive state and anactive state when depressed by the user. When the housing is comfortablyreceived within the palm of the user's hand, the left click button isreadily accessible by the finger tip of the index finger of the userwhile the right click button is readily accessible by the finger tip ofthe middle finger of the user.

According to the present invention, the input device 10 is enhanced byproviding pressure sensitive switches 18 on the housing 12 for access bythe finger tips of the user when the housing is supported within thepalm of the user's hand as in the conventional use of a computer mouse.In preferred embodiments two pressure switches 18 are provided with onebeing located for ready access by the middle finger of the user so as tobe located near the right click switch 16, while the other pressuresensitive switch 18 is located at a side of the housing 12 for readyaccess by a thumb of the user.

In some embodiments of the input device 10 only a single pressuresensitive switch 18 is provided in combination with a two state buttonof the mouse so that the pressure sensitive switch can be arranged toselect one item from a plurality of selection items. The two state rightclick or left click switch in combination therewith can then be used toconfirm entry into the computer of the item selected using the pressuresensitive switch.

In further embodiments, the two pressure switches may be provided aloneor in combination with a two state button to permit various combinationsof selections to be made as described further herein. In yet furtherembodiments, only a single pressure switch may be provided and operatedsimilarly to some of the embodiments described herein.

In each of the preferred embodiments, an input device is provided havingboth a first switch and a second switch arranged to generate respectivecontrols signals when depressed by the user. In some embodiments onlythe second switch comprises a pressure sensitive switch while the firstswitch comprises a two state button. For increasing the range ofselection items to be selected, both the first and second switchespreferably comprise pressure sensitive switches in combination with yeta further switch having a two state operation.

In each instance where the computer includes a plurality of sequentialselection items to be selected in a range from a first selection to alast selection item, each pressure sensitive switch is arranged togenerate continuous pressure values in identifiable discrete pressureranges different from one another corresponding to different pressuresbeing applied by the user in depressing the pressure sensitive switch.Each discrete pressure range of the pressure sensitive switchcorresponds to one of a list of items to be selected on the computerwhereby increasing applied pressure to the pressure sensitive switchadvances the selection item being selected towards the last selectionitem while reducing pressure applied to the pressure sensitive switchreturns the selection item being selected towards the first selectionitem. Depressing the other pressure sensitive switch or one of the twostate buttons of the input device permits confirmation of the selectionitem to be entered to the computer at any time.

The electronic circuitry can be arranged to perform various tasksdepending upon the configuration thereof. In some arrangements whenthere is provided one or more pressure sensitive switches and one ormore two state switches, at least one of the switches in someembodiments is arranged to generate an advancing control signal, whenthe pressure sensitive switch is momentarily depressed or tapped, whichadvances selection of one group through a plurality of groupsdesignating a plurality of sequential selection items among each group.The tapping of the switch thus provides a coarse selection processwhereby the general proximity of an item to be selected can be reachedquickly by sequentially selecting groups of items. Once the proper grouphas been selected which locates a desired selection item to be selectedtherein, application of continuous pressure to one of the pressuresensitive switches permits selection of the desired item from the listof selection items within that selected group. In this instance, twopressure switches are preferably provided in which tapping ormomentarily depressing the pressure sensitive switches by the usergenerates respective advancing control signals to advance selection ofone group from the plurality of groups in opposing direction relative toone another.

In embodiments where two pressure sensitive switches are provided, andit is desired to navigate through a plurality of cascading levels ofsequential selection items, the range of pressure values of eachpressure sensitive switch preferably corresponds to the selection itemsof a given level in which the pressure sensitive switch associated witheach successive level alternates between the two switches. Accordinglyby varying the application of pressure to a first one of the switches,the user can select one of the selection items within a first level. Byswitching application of pressure to the other pressure sensitive switcha subsequent level of selection items is selected and navigated throughby varying the pressure applied to the next pressure sensitive switch.In a continuing alternating manner, applying pressure to the opposingswitch causes a selection to be made within a respective level to permitthe selection to proceed to the next level of selection items.

In yet further embodiments, the pressure switches can be combined withthe various functions of two state buttons or scroll wheels on a typicalcomputer mouse or other suitable input device for a computer to permitother combinations of computer control signals to be generated forcontrolling a computer in yet further applications.

Is some embodiments of the input device, the pressure switch may beoperable in a first mode in which a variable function associated withthe pressure switch is arranged to be varied in a first directionresponsive to increased pressure applied to the switch and second modein which the variable function is arranged to be varied in a seconddirection opposite to the first direction increased pressure applied tothe switch. As an increase in pressure applied by a user offers morenatural control to in this instance may be controlling which direction avariable function is varied or a selection is scrolled when an increasein pressure is applied. The may initially apply pressure with theselection or variable function beginning at an initial selection suchthat increasing pressure results in the selection or function beingadvanced in a forward direction. If the desired item to be selected, orthe desired value of a function is passed by, the user has the option toconvert the mode of the pressure switch so that further increases inpressure cause the item to be selected or the desired function to bevaried in an opposing rearward direction back towards the initialselection.

Conversion of the mode may be accomplished by contacting an auxiliaryswitch, for example a second pressure switch or a two state buttonprovided in association with the first pressure switch on a commonhousing. Alternatively, the mode of the pressure switch can be convertedto reverse the direction of selection or the direction of variation ofthe variable function by altering the method of contact to the pressureswitch itself. In this instance, a continuous pressure applied to thepressure switch can function to vary the variable function in thedirection of the current mode, however a momentary pressure applied tothe pressure switch instead acts to convert the mode of the pressureswitch to reverse the direction that the variable function is variedwith increasing pressure applied to the pressure switch.

In a further embodiment, a pressure switch can be provided incombination with a tracking mechanism of the input device, for examplethe tracking mechanism of a computer mouse which is arranged to trackmovement of the housing relative to a supporting surface upon which itis supported. In this instance, the pressure switch and the trackingmechanism can be arranged to each controllably vary a respectivevariable function simultaneously with one another. The simultaneouslycontrol of the variation of two variable functions or two items to beselected from respective sequential lists or groups is particularlyuseful when it is desirable to control two different manipulations of anobject in a virtual desktop environment. Examples of manipulations ofthe object can include translation of an object, rotation of an object,or a zoom function to zoom the object. In preferred arrangements, thetracking mechanism of the input device controls translational movementof the object within its environment while the pressure switch functionsas noted in previous embodiments above to controllably vary a rotation,a zoom or other variable selection relating to the object.

The pressure switch in this instance, is arranged to function similarlyto previous embodiments such that an increase in pressure applied to thepressure switch is arranged to controllably vary one of the variablefunctions in one direction and a decrease in pressure applied to thepressure switch is arranged to controllably vary said one of thevariable functions in an opposing second direction. Accordingly, in afirst mode, the object can be zoomed out or rotated clockwise whenincreasing pressure is applied to the pressure switch and in a secondmode, the object can be zoomed in or rotated counter-clockwise whenpressure applied to the pressure switch is decreased.

To enhance control of the tracking mechanism, a second switch in theform of a second pressure switch or a two state button can be arrangedto fix the variable function associated with the pressure switch uponactivation of the second switch so that variations in pressure appliedwhen displacing the housing using the tracking mechanism will no longervary the other variable function once it has been set to the desiredsetting.

When there are two pressure switches in addition to the trackingmechanism both pressure switches can be associated with the same one ofthe two variable functions to be simultaneously controlled. In thisinstance, one of the pressure switches can be arranged such that anincrease in pressure controllably varies the variable function oradvances a selection in a first forward direction while the otherpressure switch is arranged such that an increase in applied pressurethereto controllably varies the same variable function or selection tovary in an opposing rearward direction.

The amount of applied pressure can be correlated to a discrete pressurerange to determine the selection or value of the variable function.Alternatively, the pressure switch can be arranged to controllably varyone of the variable functions such that any pressure applied advances aselection or varies the value of the function in one direction andvariation in pressure applied to the pressure switch corresponds to therate of variation of the variable function. An increase in pressure thusincreases the rate of change of the value of the variable function inone direction.

When there is provided a plurality of items or values to be selected arethe values are organized in groups, as noted above, a coarse selectioncan be provided to first select which group the desired item or valuebelongs followed by a fine selection to select one particular valuewithin the group. In some embodiments the pressure switch can bearranged to controllably vary the variable function through a range ofvalues by selecting between the different groups when continuouspressure is applied such that each amount of pressure applied isidentified with a given discrete pressure range associated with arespective one of the groups. The same pressure switch can then bearranged to vary the variable function among the selections within theselected group by momentarily applying pressure to the pressure switch.In this instance, each momentary pressure applied corresponds to aprescribed incremental increase in the value of the function to beselected in one direction.

Instead of relying on a continuous pre a momentary applied pressure, oneof the auxiliary switches on the input device can instead be arranged toconvert the pressure switch between the coarse mode and the fine mode.In each mode the pressure switch can be arranged as in previousembodiments to controllably vary a desired variable function. Thepressure switch may identify the pressure applied with a respectivediscrete applied by the with each increasing pressure rangecorresponding to an incremental increase in the function beingcontrolled. The incremental changes to the variable function associatedwith each discrete pressure range of applied pressure are greater in thecoarse mode than in the fine mode.

Alternatively, in both the coarse mode and the fine mode, the pressureswitch can be arranged to controllably vary one variable function suchthat pressure applied advances a selection or varies the value of thefunction in one direction and variation in pressure applied to thePressure switch varies the rate of variation of the variable function.In this instance the rate of change of the variable responsive todesignated applied pressures is greater in the coarse mode than in thefine mode.

When used in an input device for an electronic device comprising aselection function, typically a single mouse button click, and an actioninitiation function, typically a double mouse button click, the inputdevice may rely on the pressure switches to generate a selection signalidentifiable as a selection by the selection function and an actioninitiation signal identifiable as an initiation of an action by theaction initiation function of the electronic device.

To accomplish this, the pressure switch can be arranged to generate afirst signal responsive to a first user interaction and a second signalresponsive to a second user interaction in which the selection signal isthen generated responsive to the first and second signals.

In typical arrangements, the first signal is arranged to be generatedresponsive to applying a pressure to the pressure switch which exceeds afirst pressure threshold. Subsequently the second signal can begenerated either responsive to applied pressure to the pressure switchfalling below a second pressure threshold or responsive to a pressurebeing released from the pressure switch within a prescribed durationfrom the first signal.

An audio signal representing a familiar mouse button click can begenerated with the second signal to confirm that a selection signal isto be generated and recognized by the selection function of theelectronic device.

When relying a prescribed duration between the first and second signalsbeing generated to determine if a selection signal is generated, anaudio signal, in the form of an error indication, can be generated ifpressure is not released from the pressure switch within the prescribedduration from the first signal to indicate that the selection signalwill not be generated.

Regarding the action initiation or action invocation function, thepressure switch can also be arranged to generate the action initiationsignal responsive to two selection signals being generated within aprescribed period of time similar to a double lick. In this instance,the selection signals can be generated by either of the methods notedabove.

Alternatively, the pressure switch may be arranged to generate theaction initiation signal responsive to a pressure being applied to thepressure switch which exceeds an upper pressure threshold which is,greater than any pressure thresholds associated with the first andsecond signals. Typically the upper pressure threshold would be adjustedby a user preference on the electronic device and would permit the upperthreshold to be set at a value which may be considerably greater thanpressure thresholds of the selection signals by a factor of 2 or more.

The design considerations of augmenting a mouse with one and two sensorsare considered herein through two experiments. In the first study weinvestigated the ideal locations for affixing pressure sensors to amouse, the methods for selecting continuous pressure values, and thenumber of pressure values that can be controlled with one sensor. Theresults of the first study show that users can efficiently controlpressure sensors with the thumb and middle-finger. The results alsoagree with previously established norms that users can comfortablycontrol only up to 6 pressure levels [11,15]. To extend the user'sability to control a larger number of pressure levels we designed twodual-pressure control techniques, switch and tap. Switch and tapfacilitate control of over 64 pressure levels and give users the abilityto control pressure in two directions. The results of a second studyshow that a technique such as tap allows users to control higherpressure levels and provide bi-directional pressure input.

The main contributions of this paper are to: 1) extend the design spaceby augmenting the mouse with pressure input; 2) describe a framework forthe design of pressure augmented mice; 3) identify strategies forcontrolling large number of pressure values with two sensors; and 4)provide a mechanism for controlling bi-directional pressure input.

We built a design framework to identify various factors that caninfluence performance with a pressure augmented mouse. The frameworkuses six attributes to characterize the factors that can influenceperformance: sensor positions, number of sensors, discretization of rawpressure values, pressure control mechanism, selection technique andvisual feedback.

Sensor Positions

Designers can add pressure sensors to a mouse in multiple differentlocations. Ideally, pressure input should not require the user tointerrupt a task or to reposition the hand to access a pressure button.Additionally, pressure control is best at the fingertips [18]. Thereforeto provide greater user control and better resolution of pressurelevels, designers should position the sensors so that they can beaccessed within the reach of the finger tips, such as on the rim insteadof the surface of the mouse. Several manufacturers such as Logitech orApple's MightyMouse™ use this approach of adding buttons to the rim ofthe mouse and within the range of the finger tips.

The primary button on a mouse is typically controlled by the indexfinger unless the mappings of the button are modified. As a result,unlike styluses or touchpads [11,15] with which pressure input isprovided by the index finger, designers should not place on a mouse apressure button in a location that interferes with the index finger.Accordingly, for easy access, higher ergonomic control and reduced taskinterruption, users should be provided access to pressure buttonsthrough the thumb, middle-finger, ring-finger or little-finger.

Number of Sensors

Most studies have investigated the use of pressure based input ondevices such as digitizers, pens or touchpads [11,15,16]. These devicesare limited in terms of adding more sensors. However, with respect tothe physical design and common usage of a mouse, we can easily affix one(uni-pressure) or two (dual-pressure) sensors onto it so that users cancontrol them simultaneously. We propose that up to two sensors can becontrolled simultaneously on a mouse, and controlling more than twosensors would strain the user.

Discretization of Raw Pressure Values

Exerting force on a pressure sensor produces a raw stream of discretenumeric integer values. The analog force exerted by the user getsconverted to a digital data stream through a manufacturer specificAnalog-to-Digital (AtoD) converter. As a result, manufacturers provide256, 512 or 1024 discrete integer pressure values. However, users cannotcontrol effectively the raw discrete values. As a result, applicationsfurther discretize the mw integer values by grouping near-by values intounique controllable pressure levels [11,15].

In stylus and pen based pressure input, studies have shown that userscan comfortably control up to 6 tl discrete pressure levels [11,15].Furthermore, users can better control forces that are less than 3N [11],Since manufacturers apply different analog-to-digital converters thereis no standard mechanism to discretize the number of pressure levels. Asa result, there are numerous methods and mappings for discretizing thenumber of controllable levels using a pressure based input [11,14].

In one reported case of pressure discretization, Ramos et al. [14]process the raw pressure values through a low-pass filter, a hysteriafunction to stabilize the raw signal, and a parabolic-sigmoid transferfunction to account for pressing on the stylus' pressure tip. As aresult there is a slow response at low pressure levels, linear behaviourin the middle levels and slow response at the high levels of thepressure range [14]. Mizobuchi et al. [11] used a linear discretizationfunction by creating equal pressure levels consisting of 0.41 N each.Ramos et al. [15] use a linear discretization function to map 1024pressure values into units with the same number of pressure values. Someexamples of different discretization methods are depicted in FIG. 2. Thediscretization function needs to take into consideration the type ofpressure sensor and the user's ability to comfortably control thepressure values.

Pressure Control Mechanism

A pressure control mechanism allows the user to iterate through a listof available pressure levels. In most pressure based interactions,pressure input is usually better controlled in one direction, i.e, whengoing upward from 0 to the highest value but not in the reversedirection. As a result, in a uni-pressure augmented mouse, the pressurecontrol mechanism is basic and simply consists of pressing down on onesensor to iterate through a limited number of pressure levels. However,it would be beneficial to devise a pressure control mechanism thatfacilitates controlling input in both directions. This mechanism can beprovided by means of some specialized hardware [16] or by augmenting themouse with more than one sensor.

Many types of interactions, such as mode switching and menu selectioncan benefit from a large number of pressure levels than what has beentypically reported. Increasing the number of accessible pressure levelsmay be possible with two sensors. We propose that pressure controlmechanisms with a dual-pressure augmented mouse consider the followingdesign goals: the user should access a larger number of pressure valuesthan with one pressure input; there should be minimal overhead when theuser switches applying pressure between the different sensors; eachpressure sensor should not extend beyond the comfortable control rangeavailable to the user; if possible dual-pressure mouse should providepressure control in both directions.

Selection Mechanism

A selection mechanism allows users to pick the required value afterusing the pressure control mechanism to hone into a pressure level.Ramos et al. [15] proposed several selection mechanisms-QuickRelease,Dwell, Stroke and Click-for stylus based pressure input. QuickReleaseoperates by quickly lifting the stylus from the tablet's surface afterreaching the appropriate pressure level. Dwell triggers the selectionafter the user maintains the pressure control over a prescribed amountof time. Stroke activates the selection mechanism after the user makes aquick spatial movement with the stylus. Click selects a level bypressing the stylus' barrel button. On a stylus, QuickRelease was shownto be the most effective selection technique [15]. However, it is notclear whether this method is appropriate for a uni-pressure anddual-pressure mouse. Furthermore, it is possible that differentselection mechanisms are required in a dual-pressure augmented mouse toallow the user to Switch between sensors.

Visual Feedback

Kinesthetic feedback alone is insufficient for adequately controllingand selecting a pressure value. Visual feedback is a necessary componentof the interaction space with pressure based input [11,15]. The mostcommon form of feedback is through a visual highlight over the activeitem that is selectable. Ramos et al. [15] investigated the effects oftwo different visual feedback conditions: full visual and partial visualfeedback, in the full visual feedback (or continuous feedback) conditionall the potential targets are visible. As the user applies pressure, thevisual indicator (typically a highlight) iterates through the list ofselectable items. In the partial feedback (or discrete feedback)condition only the selected target is visible, in a similar setup,Mizobuchi et al. [II] investigated the effect of continuous and discretevisual feedback. In both the above described studies, users performedbetter with the continuous feedback condition.

Study of a Pressure Augmented Mouse

To investigate the influence of the above factors on performance wecarried out two studies. The first study informs the design choice ofdifferent sensor positions and selection mechanisms. The second studyexamines the effects of uni- and dual-pressure control mechanisms onperformance.

Hardware Configuration and Discretization Function

Both our studies used an optical mouse with pressure sensors mounted onits rim (FIG. 1). The sensors (model #1ESF-R-5L, from CUI Inc.) couldmeasure a maximum pressure value of 1.5 Ns. Each sensor provided 1024pressure levels. The application was developed in C# and the sensor wascontrolled using the Phidgets library [13]. The experiments wereconducted in full-screen mode at 1024×768 pixels on a P4 3.2 GHz WindowsXP OS.

We first carried out a pilot study with three subjects to compare threedifferent pressure discretization functions: a linear function, aquadratic function centered at the lowest pressure value and a quadraticfunction centered at the middle pressure value (DFI, DF2, DF3 in FIG.2).

With the linear function we observed that users controlled lesseffectively the lower pressure values than the higher values. We foundthat users were fastest with the quadratic function centered at thelowest pressure values. In this discretization method, targets in thelower range contained more pressure units than those in the higherrange.

Performance Measures

The experimental software recorded trial completion time, errors andnumber of crossings as dependent variables. Trial completion time (MT)is defined as the total time taken for the user to apply the appropriateamount of pressure and select the target. The number of crossings (NC)is defined as the number of times the cursor enters or leaves a targetfor a particular trial. The software records an error (E) when theparticipant selects a location which is not a target. The trial endedonly when the user selected the right target, so multiple errors werepossible for each trial. While MT and E give us an overall success rate,NC provides information about the level of control achievable using eachof the different pressure control mechanisms. Participants were alsoasked in an exit questionnaire to rank the different selection andpressure control techniques.

Experiment 1 Methods

The goal of this experiment was to examine differences in performancewith different sensor locations and different pressure selectionmechanisms. The experiment was also designed to examine differences inselection time and accuracy at different pressure levels. We adapted theexperimental design used in [15] to this study.

Participants

Nine participants (5 males and 4 females) between the ages of 19 and 25were recruited from a local university. All subjects had previousexperience with graphical interfaces and used the mouse in their righthand.

Task and Stimuli

We used a serial target acquisition and selection task similar to thetask in [15]. Participants controlled the movement of a red pointeralong a vertical line through a sequential list of items using pressureinput. 900 pressure values were discretized in a quadratic manner (DF2in FIG. 2). A set of consecutive rectangles were drawn along the line'slength. During each trial a target was coloured in blue. The user's taskwas to apply sufficient pressure to move the red pointer into the bluetarget. We provided complete visual feedback to the user by highlightingthe items in teal when the user iterates through them. The user invokesa selection mechanism for choosing an item once the cursor is at therequired pressure level. The color of the target changed to yellow whenthe user selected it. The system generated an audio sound to givefeedback when the task was completed correctly.

Experiment 1 Procedure and Design

The study used a 5×3×3×4 within-participants factorial design. Thefactors were:

-   -   Pressure Levels: 4, 6, 8, 10, 12.    -   Selection Mechanism: Quick Release, Dwell, Click.    -   Sensor Location: Right, Left, Top.    -   Relative Pressure Distance: 395, 535, 675, 815.

The order of presentation first controlled for sensor location and thenfor selection Mechanism. Levels of the other two factors were presentedrandomly, We explained the selection mechanisms and participants weregiven ample time to practice the techniques at the beginning of theexperiment. The experiment consisted of three blocks with each blockcomprising of two repetitions for each condition. With 9 participants, 5pressure levels, 3 selection mechanisms, 3 sensor locations, 4distances, 3 blocks, and 2 trials, the system recorded a total of(9×5×3×3×4×3×2) 9720 trials. The experiment took approximately 60minutes per participant.

Selection Mechanisms

Three selection mechanisms were tested for the uni-pressure augmentedmouse: Quick Release (QR), Dwell and Click.

Quick Release: This technique is similar to the one designed in [15]. InQR, once the user reaches the desired target they select it by quicklyreleasing the linger off the pressure sensor.

Dwell: This technique is similar to the one designed in [15]. In thismethod the user maintains the cursor within the target for apredetermined amount of time. We use a delay period of 1 sec to triggerthe selection.

Click: In this method the user iterates to the desired target and clickson the left mouse button to select the item.

Sensor Locations

Three sensor locations were tested in the experiment: top, left andright. The top sensor can be easily acquired by the user's middlefinger. The left sensor is accessible by the user's thumb and the sensorin the right location is accessible with the ring or little finger. Wedid not select a sensor location that requires using the index finger asit hampers the click selection technique. The mouse was equipped withonly one sensor and the experimenter changed the location to match thecorresponding experimental condition.

Relative Pressure Distances (Distance)

In each trial a target appeared in one of four different relativepressure distances, 395, 535, 675, and 815. The relative pressuredistance is the number of pressure units from the start of the pressurerange (FIG. 3).

Results of Experiment 1 Completion Time

We used the univariate ANOVA test and Tannhane post-hoc pair-wise tests(unequal variances) for all our analyses. To make the data conform tothe homogeneity requirements for ANOVA we used a natural-log-transformon the completion time. Results showed main effect of selectiontechnique, sensor location, pressure-levels and target-distances (allp<0.01) on trial completion time with F_(2,16)=20.05, F_(2,16)=4.57,F_(4,32)=113.06, and F_(3,24)=21.655 respectively.

Post-hoc pair-wise comparisons of pressure-levels yielded significantdifferences (all p<0.01) in trial completion times for all pairs exceptbetween pressure-levels 4 and 6. Users were fastest when the pressurelevel was 4 and slowest at pressure level 12.

Post-hoe pair-wise comparisons of selection techniques yieldedsignificant differences (all p<0.01) in trial completion times for allpairs. Participants were fastest with click, followed by dwell and QR.FIG. 4 (left) shows the mean completion time of each technique perpressure level.

Post-hoc pair-wise comparisons of sensor location yielded significantdifferences (p<0.01) in trial completion times between right-and-top andright-and-left sensor pairs. Participants were faster with the sensor inthe top sensor location followed by left and then right. FIG. 4 (right)shows the mean completion time for each sensor location across thedifferent pressure levels.

Post-hoc pair-wise comparison of target distance yielded significantdifferences (all p<0.01) in trial completion times for all pairs excepttargets at relative distance D1 and D2.

Crossings and Errors

The average number of crossings per trial across all conditions was 1.3(standard error=0.022). ANOVA tests revealed a significant effect ofselection technique on number of crossings (F_(2,16)=11.35, p<0.001).Post-hoc pair-wise comparisons of selection techniques yieldedsignificant differences (all p<0.001) in number of crossings for allpairs. Click resulted in the least number of crossings, followed bydwell and QR. Our tests did not show significant effect of sensorlocation on crossings. FIG. 5 shows the average crossings per pressurelevel for each technique (FIG. 5 left) and sensor Location (FIG. 5right),

The average number of misses across all conditions was 0.23 errors pertrial (standard error—0.007). With respect to selection techniques dwellhad the least number of errors (0.01) followed by Click (0.26) and QR(0.42). For sensor locations the ordering was top (0.22), left (0.23)and right (0.25). The ordering of errors for different pressure levelswas 4 (0.09), 6 (0.14), 8 (0.24), 10 (0.28) and 12 (0.41). The orderingof errors for target position was D2 (0.22), D 1 (0.24), D3 (0.23) andD4 (0.23).

Subjective Ranking

In the exit questionnaire we asked participants to rank the differentselection techniques and sensor locations in terms of preference. Mostparticipants preferred click (6 first places, 3 second places) followedby dwell (2 first, 4 second and 3 third) and quick-release (I first, 2second and 6 third). Most participants preferred the left location forcontrolling the pressure sensor (6 first, 3 third) followed by top (3first, 5 second and 1 third) and then right (4 second and 5 third). Wealso asked participants to rank the different selection techniques foreach sensor location. The results were similar to the overall preferenceof the selection techniques.

Discussion Selection Technique

The results of our study show that participants were fastest, had ahigher level of control (as indicated by the number of crossings) andhighly preferred the click selection technique. This result is differentfrom that reported by Ramos et al. [15] for a pen, in which they foundperformance with Quick-Release to be the fastest. There are severalpossible reasons for this finding. The proximity of the button to thepressure sensor and the resulting ergonomics made it easy for theparticipants to use their index-finger to click the left mouse button.Additionally, users reported being more comfortable clicking to invoke aselection, as this is common with mouse input. However, we also notice alarge number of errors with click. One possible explanation is thatclicking on the mouse button requires support from the other fingerssuch as the thumb which can adversely affect the pressure input (ourresults show that the largest number of errors with click occur when theuser interacts with the left sensor—thumb).

Our results indicate the dwell is a relatively good selection techniqueas seen by the significantly lower number of errors. This is in-linewith the results reported by Ramos et. al. [15]. Users completed thetask with higher accuracy in dwell than in click and quick-release. Oneexplanation for this is that with dwell users can ensure the correctobject is selected by dwelling on it for a sufficiently long period oftime. However, with dwell, if users cannot reach the appropriate level asignificant amount of adjustments are made. This is noticeable in thehigher number of crossings, particularly with the larger pressurevalues. Additionally, in our study dwell triggers a selection after a Isecond delay, It is possible that with a smaller delay users performequally well with dwell as they do with click, However, smaller delaysmay result in a larger number of errors and possibly a much highernumber of crossings.

Interestingly, unlike results from prior studies, quick release resultedwith the poorest performance values for completion times and number ofcrossings. One possible explanation is that unlike pen basedinteraction, lifting individual fingers off the mouse is not as naturalor as easy as lifting a pen from a Tablet's surface, Furthermore, thereis only a limited range of movement for individual fingers and liftingthem separately from the surface of the mouse requires considerableeffort.

Sensor Location

We found that participants were significantly slower with the rightsensor location and preferred it the least of all the locations. Ourresults do not favor the design choice of mounting pressure sensors tothe right side of the mouse. This finding counters the growing trendamong commercial manufacturers (MightyMouse™) that mount sensors orbuttons that are accessible with the ring or little finger.

Interestingly, the interaction effects between number of pressure levelsand sensor location suggest that different sensor locations are bettersuited for controlling varying degrees of pressure levels. For smallerpressure levels users committed a smaller number of errors with the topsensor (middle finger) while at larger pressure-levels users committedfewer errors with the left sensor (thumb).

Pressure-Levels

Results on speed, number of crossings and accuracy, indicate thatperformance degrades rapidly when the number of pressure-levelsincreases beyond 6. This result is supported by prior studies onpen-based interfaces that suggest it is difficult to control more than6±1 pressure levels [11,15]. In experiment 2, we extend the design ofthe uni-pressure augmented mouse by affixing an additional pressuresensor to determine if this limit can be extended.

Dual-Pressure Input

Augmenting the mouse with one pressure sensor limits the number ofaccessible pressure levels. Many applications such as zooming-in/out ofa workspace, modifying the brush thickness in a drawing application oriterating through a long list of items can benefit from interacting witha large number of pressure levels. Additionally, a uni-pressureaugmented mouse does not facilitate bi-directional input. In ourcontext, bi-directional input refers to the user's ability to control,equally well, pressure input when pressing (forward) and releasing(backward) the sensor. From our observations (and prior work [15]),continuous pressure input with one sensor affords a much higher degreeof forward control over backward control. These limitations led to thedesign of pressure control techniques, with two sensors.

The dual-pressure augmented mouse uses one pressure sensor that iscontrolled by the middle-finger and the other controlled by the thumb,Results from experiment 1 suggest that users apply a coarse grainedmovement to get closer to a target and then apply a finer movement to“coast” onto the target. This observation led to the design ofswitch-to-refine and tap-and-refine.

Switch-to-Refine

Switch-to-refine (or switch) allows users to switch between two sensorsto control a large range of pressure values. In switch-to-refine onesensor is considered as primary and the other as secondary. The range ofpressure values are divided such that users apply pressure on theprimary sensor to access a coarse-level set of pressure values, each ofwhich is interleaved by a range of fine-level pressure values (FIG. 6).In this pressure control mechanism the participant uses the primarysensor to coarsely jump through the coarse-level items and switches tothe secondary sensor to control and navigate in a finer manner throughthe set of values between the coarse-level items. To assist the user,the primary sensor does not respond while the user is refining theirselection with the secondary sensor. Once the user reaches theappropriate pressure level they click on the left mouse button to selectthe item.

If the total number of selectable items is 48, we can group the itemsinto eight coarse-level values each containing six fine-level items (seeFIG. 6). To select the 15^(th) item the users starts with the primarysensor and applies pressure to reach the 3^(rd) coarse-level item (whichis item 13 in the entire range). This is followed by switching to thesecondary sensor to navigate through each of the fine-level items incoarse-level item number 3. As a result, the secondary sensor allows theuser to navigate through each of the 6 items from item-13 to 18. Toselect the 15^(th) item the user applies 3 levels of pressure with thesecondary sensor. “This technique allows users to select n×m levelswhere n and m are the maximum number of pressure values that users cancontrol with the primary and secondary sensors, respectively.Unfortunately, switching from one sensor to the next creates additionaloverhead in switch-to-refine. Furthermore, switch-to-refine does notfacilitate bidirectional pressure input.

Tap-and-Refine

Tap-and-refine (tap) categorizes pressure values into coarse-level andfine-level items similar to that in switch-to-refine. However, theinteraction method in controlling the pressure input is different. Theuser iterates through the coarse-level items by tapping (quick press andrelease within 60 ms) onto the primary sensor which sets the pressurecursor at that level. Once the pressure cursor is at a givencoarse-level, the user accesses the finer levels by pressing onto thesame pressure sensor. For example, to access the 15^(th) item, the usertaps 3 times. On the third tap the user holds down on the primary sensorto iterate to the 15^(th) item and then clicks on the mouse button toselect it. Interacting with each sensor allows the user to move throughthe items in one of two directions (upward from 0 to maximum with theprimary sensor, or downward from maximum to 0 with the secondarysensor). As a result of bidirectional control with tapping, users caneasily adjust any overshoots that results from tapping too quickly.

Experiment 2 Methods

In this experiment we evaluate the various pressure control mechanismswe designed and investigate the benefits and trade-offs of uni- anddual-pressure input.

Participants and Apparatus

Eight paid volunteers (7 males and 1 female) between the ages of 21 and26 participated in experiment 2. All subjects had previous experiencewith graphical interfaces and used the mouse in their right hand. Theapparatus was similar to that of experiment 1 with the difference thatwe used a pressure augmented mouse with two sensors.

Experiment 2 Procedure and Design

The experimental task and the performance measures collected were thesame as for the previous experiment.

The study used a 4×3×4 within-participants factorial design. The factorswere:

-   -   Pressure Levels: 4, 12, 16, 64.    -   Pressure Control Technique: Switch-to-Refine, Tap-and-Refine,        Normal.    -   Relative Pressure Distance: 395, 535, 675, 815.

All conditions were presented randomly. Participants were explained howthe selection techniques worked and were given ample time to practicethe techniques at the beginning of the experiment. The experimentconsisted of three blocks each with five repetitions per condition.

Pilot trials showed that users were unable to control 64 levels with theNormal technique. So we only tested it for pressure levels 4, 12 and 16.With 8 participants, 4 pressure levels for switch and tap and 3 pressurelevels for normal, 4 distances, 3 blocks, end 5 repetitions per block,the system recorded a total of 5280 trials per participant. Theexperiment took approximately 60 minutes per participant.

Pressure Control Techniques

We evaluated switch-to-refine and tap-and-refine (described above) andcompared these to Normal technique used in Experiment I which relied ononly one pressure sensor. All techniques used the click selectionmechanism used in experiment 1.

Results of Experiment 2 Time

The overall mean completion times across all conditions was 1.57 s(standard error=0.044 s). To make the data conform to the homogeneityrequirements for ANOVA we used a natural-log transform on the completiontime. Results show a main effect of Control Mechanism andPressure-levels on trial completion times with F_(2,14)=18.46, (p<0.01)and F_(3,21)=178.106, (pa0.01) respectively.

Post-hoc pair-wise comparisons of pressure-levels yielded significantdifferences (all p<0.o1) in trial completion times for all pairs exceptbetween pressure-levels 12 and 16. Users were fastest when the pressurelevel was 4 followed by 12, 16 and 64.

Post-hoc pair-wise comparisons of control-mechanisms yielded significantdifferences (all p<0.01) in trial completion times between Tap andNormal and Tap and Switch. We did not find any significant differencebetween Normal and Switch-to-Refine techniques. Users were fastest withTap followed by Normal and Switch, FIG. 7 shows the mean completion timeof each technique per pressure level.

Crossings and Errors

The average number of crossings per trial across all conditions was1.053 (standard error=0.0735). ANOVA tests reveal a significant effectof control mechanism on number of crossings (F_(2,14)=19.101, p<0.001).Post-hoc pair-wise comparisons of control mechanisms showed that Tap hadsignificantly (all p<0.001) less number of crossings than all othertechniques. Pressure-levels also had a significant effect on number ofcrossings (F_(3,21)=39.764, p<0.001). Post-hoc pair-wise comparisonsshow that pressure-level 4 had significantly less crossings than levels12, 16 and 64. However, we found no significant difference in crossingsbetween all the other levels. FIG. 8 shows the average crossings perpressure level for each.

The average number of errors across all conditions was 0.25 errors pertrial (standard error=0.01). With regard to control mechanisms tap andswitch had 0.17 errors followed by Normal (0.47). The ordering ofaverage number of errors for different pressure levels was 4 (0.12), 12(0.31), 16 (0.32) and 64 (0.25).

Discussion

The results of the second experiment show that the mouse can beaugmented with more than one pressure sensor to extend the user'spressure control range. In the following sections we discuss thebenefits and limitations of the various pressure control mechanisms wedeveloped, application areas that can benefit from a pressure augmentedmouse and summarize the main lessons for practitioners.

Pressure Control Mechanisms

We observed various pressure control strategies with the uni-pressureand dual-pressure augmented mouse.

Uni-Pressure Control Strategies

The experimental software recorded continuous time and pressure valuesfor each trial. A typical trace of a user's selection task when usingthe click mechanism is shown in FIG. 9. Users' action can becharacterized by two steps: First a coarse-grained pressure input to getcloser to the target and then a fine-grained precision movement toselect the target. In the coarse-grained movement users apply instantlyand rapidly a pressure amount to get in the range of the desiredpressure value. However, to select the appropriate target, users thencontrol more carefully the pressure input up to the target.

More precisely, we notice that once users get within the vicinity of thetarget they take approximately between 150 and 300 ms to refine theirpressure movement to select the target. This is often the time it takesthe user to feel confident that they have the correct pressure value andmomentarily switch their attention to the index finger for clicking thebutton. Further enhancements in this fine-grained stage will improveperformance of the click selection technique and possibly allow the userto select more than 6 pressure levels.

Dual-Pressure Control Strategies

With dual-pressure strategies users were able to better controldifferent pressure levels using tap-and-refine than switch-to-refine.This was a result of the several factors. With tap, users can controlpressure levels bi-directionally. As a result, overshoots can be easilycorrected. Additionally, since with tap users depend on tapping to gettoward the vicinity of the target, users have a higher degree of controlover the coarse-level items. The fine-level items require furthercontrol which can easily be handled if the set of fine-level itemscontain less than six items, Switch is partially restricted by thenumber of levels controllable with each sensor, In our study we comparedthe two techniques at 64 discrete levels. These were separated into 8×8discrete levels. As a result, adding more levels to any of the twosensors would show significant performance decreases with switch.

The tap in Tap-and-Refine may be replaced by a simple button. The designwould need two additional buttons (one for each direction) and onepressure sensor to work effectively. However, using the standard rightor left-click buttons would interfere with the click selection mechanismand other mouse functionalities. Further, the context switch that wouldensue switching between the button and the pressure sensor would furthercontribute to reduced performance of the technique. Analysis of our logfiles suggest that typical tap times are about 50 to 80 ms which seemsfaster that the button click times reported in [8], However, furtherresearch is needed to investigate alternatives to Tap-and-Refine.

Applications

A pressure augmented mouse can enhance interactivity in a number ofdifferent applications,

Integrated scaling and parameter manipulation. Ramos et al. [14]proposed a fluid pen-based interaction technique, Zliding thatintegrates scaling and parameter manipulation. In Zliding users controlthe scaling factor by applying pressure at the stylus' tip and delegateparameter manipulation to the stylus' x-y position. Tap-and-refine canbe modified to accommodate the design goals of an integrated scale andparameter manipulation technique. In tap, the parameter manipulationwould be assigned to the coarse-level movement of tapping onto thepressure button. The scale factor would be relegated to the holding-downaction in the tap.

Mode switching. Many applications require that users switch betweenmodes rapidly [8]. In games for instance, it is critical that usersswitch modes quickly to access a weapon or some other tool. In drawingapplications, a significant amount of work takes place in small localregions of the workspace. Drawing applications require that users accessdifferent options on palettes in the application for such tasks asmodifying the thickness of the pen of changing a color. Pressure buttonscan allow users to select a mode without making significantdisplacements in the application.

Pressure Menus. Pressure menus could be designed in a similar manner topolygon marking menus [21]. On the spot, users can trigger and interactwith a large menu. Using tap users can iterate through an infiniteamount of menu values and refine their selection as needed.

Design Recommendations

There are several lessons that designers can take from our experiments:

-   -   Place pressure buttons so that they are accessible by the        middle-finger and the thumb.    -   Consider mechanisms for selecting pressure values based on mouse        button clicks.    -   Use dual-pressure mechanisms for increasing the selectable range        of pressure levels and modes.    -   Consider tap-and-refine as a control mechanism for providing        bi-directional pressure input on a large number of pressure        levels.

Augmenting a mouse with pressure based input poses several designchallenges, some of which we addressed in this paper. Results of thefirst experiment show that pressure buttons are best controllable by themiddle-finger and the thumb. The first study also confirmed that userscan comfortably control a limited number of pressure levels with onepressure button. Additionally, the uni-pressure augmented mouse did notfacilitate bi-directional pressure input. The limitations of auni-pressure augmented mouse led to the design of a dual-pressureaugmented mouse along with two interactive mechanisms, tap-and-refineand switch-to-refine, to control pressure levels. The results of thesecond study showed that with tap-and-refine users can comfortablycontrol a large number of pressure levels. Furthermore, withtap-and-refine users can provide pressure input in a bi-directionalmanner.

Discretization Functions

As described above, the pressure sensitive switch is arranged todistinguish among pressure values in discrete pressure rangescorresponding to different pressures being applied by the user bydividing the entire range of pressure values into discrete units with adiscretization function. The entire range of pressure values can bedivided into 8 or fewer discrete units in some embodiment, however whenusing certain discretization functions, the pressure values may bedivided into 8 or many more discrete units. In each instance, thediscretization function is arranged to produce a limited number ofdiscrete levels to facilitate the control of raw pressure valuesobtained from the pressure switch. In other words the discretizationfunction converts analog pressure values to a limited number of discretelevels.

The discretization function can be linear, quadratic or fisheye.

When the function is linear, the function is given by the followingequation,

$y = {{floor}\left( \frac{x*l}{R} \right)}$

where x is the raw pressure value from the pressure switch, I is thenumber of pressure ranges, and R is the total number of raw pressurevalues.

When the function is quadratic, the function is given by the followingequation,

$y = {{floor}\left( \frac{\left( {x^{2}*l} \right)}{R^{2}} \right)}$

where x is the raw pressure value from the pressure switch, I is thenumber of pressure ranges, and R is the total number of raw pressurevalues.

The fisheye discretization function is particularly advantageous forincreasing the controllable number of discrete pressure units as it isarranged to divide the range of pressure values into discrete pressuresunits such that a currently selected one of the discrete pressure unitsis arranged to be larger between respective upper and lower pressurevalue limits than remaining non-selected discrete pressure units.Accordingly, once the pressure switch is depressed by a given pressurevalue, the correspondingly selected discrete pressure unit will remainselected despite small variations in the applied pressure value. Thefisheye function is given by the following equation,

$y = \left\{ \begin{matrix}{{{floor}\left( \frac{\left( {x - r} \right)*\left( {l - 1} \right)}{R - r} \right)} + 1} & {x > {r - \frac{R - r}{l - 1}}} \\0 & {x \leq {r - \frac{R - r}{l - 1}}}\end{matrix} \right.$

where x is the raw pressure value from the pressure switch, I is thenumber of pressure ranges, r is the fisheye radius, and R is the totalnumber of raw pressure values.

Recently, several studies have reported the benefits of pressure-basedinteraction as an alternative input channel [24, 27-29]. Ramos et al.[28,29] explored the design space of pressure-based interactions withstyli. Their results revealed that adequate control of pressure valuesis tightly coupled to a fixed number of discrete pressure levels(maximum of six levels). Mizobuchi et al. [26] investigated accuratecontrol of pressure exerted on a pen-based device and showed that userscan better control forces that are divided into discrete levels andsmaller than 3N. Cechanowicz et al. [24] investigated augmenting a mousewith one or two pressure sensors and showed that with one sensor userscan control between 8 and 10 discrete levels.

One common outcome reported by previous studies is the high number oferrors that result from pressure-based input [24,29]. As a result,pressure input may not be highly practical as a reliable inputdimension. This limited ability to properly control pressure has made itdifficult to introduce pressure input to facilitate tasks that requiremultiple levels of pressure control such as in menu navigation [24],scrolling, and high-precision zooming [28].

The discretization method that researchers employed for dividing thepressure range into discrete units or levels is an important aspect ofthe pressure-based systems in previous studies. However, sincemanufacturers of pressure-sensing devices apply differentanalog-to-digital (AtoD) converters there is no standard mechanism todiscretize the number of pressure levels. As a result, there are manymethods and mappings for discretizing the number of controllable levelsusing a pressure-based device [24,27,29]. Ramos et al. [28] andMizobuchi et al. [27] used a linear discretization function, while Ramoset al. [29] used a parabolic-sigmoid discretization function thatresulted in a slow response at low pressure levels, linear behaviour inthe middle levels, and a slow response at the high levels of thepressure range. Cechanowicz et al. [24] used a quadratic discretizationfunction that allocated larger pressure ranges at the lower levels andsmaller pressure ranges at higher levels.

Studies investigating user control of pressure input have reportedtime-accuracy trade-offs of, on average, over 30%, when interacting witha large number of pressure levels. To increase the level of control withpressure input, we de-signed and evaluated four different discretizationfunctions: linear, fisheye, visual fisheye, and clustered. The fisheyediscretization dynamically modifies the range of pressure values basedon the position of the pressure cursor. Our results show that a fisheyefunction results in significantly lower error rates and a lower numberof crossings than have been reported in the literature. Furthermore, thefisheye function improves control without compromising speed. We discussthe findings of our study and identify several design recommendationsfor integrating pressure control into common interface tasks.

In this paper we present the design of PressureFish, a fish-eyediscretization function (see FIG. 11) and compare it to a variety ofdiscretization methods proposed in the literature. We carry out ourinvestigation on a pressure augmented mouse [24]. Our results show thatthe fisheye function increases accuracy without compromising speed. Forexample, at 10 pressure-levels with a fisheye function, users aresignificantly more accurate with 78% accuracy compared to 54% for linearand require significantly less target crossings (i.e., overshooting thetarget before acquiring it) with an average of 0.4 crossings per trialcompared to 0.7 for linear. Overall, by using the fisheye discretizationfunction users are able to exhibit better control of pressure input.

Discretization Functions

The analog force exerted by the user on a pressure sensor is convertedto a digital data stream through a manufacturer-specific AtoD converter.As a result, manufacturers provide 256, 512, or 1024 discrete integerpressure values. However, users cannot effectively control these largenumbers of discrete values. Applications further discretize the rawinteger values by grouping adjacent values into unique controllablepressure levels [24,29]. Here we describe the various discretizationfunctions (henceforth referred to as ‘function’) we evaluated in thisstudy.

In our descriptions we use the following variables: x—the raw pressurevalue from the sensor; I—the number of pressure levels the space isdivided into; R—the total number of raw pressure values from thepressure sensor.

Linear Discretization: A linear function (L) partitions the entirepressure space into equal units. For instance, a pressure space of 600units (R=600) divided into 10 levels (I=10) would produce levelsconsisting of 60 pressure units each. The linear function is given by(formula)

Numerous studies have reported using a linear function to controlpressure input [23,24,26].

Clustered Discretization:

Some groups [24,29] have used functions that assign more pressure levelsto the middle range of the pressure space by hand-picking various designparameters like the starting pressure unit for each level and the numberof pressure units for each pressure level. Rather than hand-pick, weused a K-means clustering algorithm to discretize the space. Users wereasked to select randomly highlighted pressure levels discretized usingthe linear function described above and a quadratic function describedby Cechanowicz et al. [24]. We collected raw pressure values for 6 users(208 trials/user) and used the K-means clustering algorithm to design anoverlapping discretization for each pressure level. Following a pilotstudy that showed no significant difference between the quadratic andlinear functions, we decided to proceed with the linear function only,to allow us to compare and contrast linear (L) with K-mean clusteredlinear (KC).

PressureFish Discretization:

This fisheye function (FE) was inspired by the fisheye distortionfunctions introduced by Furnas [25] and applied to fisheye menus [22].The idea of a fisheye function is to make the area near the point ofinterest highly visible. This results in a distortion with a variableamount of space reserved for the various elements in the pressure space.Items further away from the focal point occupy less space, while itemscloser to the focal point occupy more space, and the item of focusitself occupies the largest amount of space. While this distortion ofthe visual space offers enhanced visibility researchers have alsore-ported targeting problems that arise from the constant change ofcontrol-to-display ratio [26].

However, the fisheye function could be particularly advantageous as adiscretization function for three reasons. First, when the pressurecursor is at the level of interest, the fish-eye function automaticallyincreases the amount of pressure values assigned to that pressure level.As a result, when the user presses the pressure sensor and fixes it to aparticular level, the selected pressure value remains selected despitesmall variations in the applied pressure value. Second, finger-tips havea tendency to exert inadvertent forces. Such forces directly affect themovement of the pressure cursor, thereby reducing the level of usercontrol. Since the fisheye function reserves sufficient space for theactive pressure item, minor forces from the tips of the finger do notsignificantly impact pressure control. Finally, since the control spaceinvolves depressing a sensor rather than moving a mouse, users are lesslikely to have targeting problems.

We use the following fisheye function (r=fisheye radius)(formula)

To effectively control the fisheye selection, several design choices arepossible. Each of the design parameters consist of modifying the valuesfor r,

R, and I given the equation above. In this study, we used values ofR=600, I consisting of values 6, 8, 10, 12, and 16, and r was assigned avalue of 120 pressure units. These values were selected based on anumber of pilots we ran before starting the final study.

Visual Fisheye Discretization:

Visual feedback is an essential element in pressure-based interaction[24, 27-29]. While the fisheye function divides the entire pressurespace into non-uniform units, the visual fisheye (VF) function uses anunderlying linear function but presents the visualization as a fisheyemenu. As a result, the users are controlling the pressure cursor using alinear function but are being led into believing that the pressure isbeing controlled using a fish-eye technique. The motivation behind thedesign of VF is that if such a technique were to be successful thendevelopers could simply enhance the visual presentation of pressureinput. We were interested in identifying whether the visual effects weresufficient to improve control without changing the underlyingdiscretization function (i.e. identify the degree of importance ofvisual feedback on pressure input).

Comparison of Discretization Functions

Our experimental goal was to examine differences in accuracy, speed andnumber of crossings using different functions. The experimental designwe used was adapted from two other studies [24,29].

The experimental software recorded trial completion time (MT), errors(E), and number of crossings (NC) as dependent variables. MT is thetotal time taken for the user to apply the appropriate amount ofpressure and select the target. NC is the number of times the cursorleaves a target after first entry for a particular trial. E is thenumber of times the participant selects a location which is not atarget. The trial ended only when the user selected the right target, somultiple errors were possible for each trial. While MT gives us anoverall success rate, E and NC provide information about the level ofcontrol achievable using each of the different pressure-controlmechanisms.

We used an optical mouse with a pressure sensor mounted on the leftside, where it is easy and comfortable to be accessed with the thumb(FIG. 11). The sensor (model #IESF-R-5L from CUI Inc.) could measure amaximum pressure value of 1.5N and provided 1024 pressure levels.However in our experiment we only used the range from 0 to 600, asearlier studies suggest that user fatigue is common at higher pressureranges [24]. The software was implemented in C# and the sensor wascontrolled using the Phidgets library. The experiment was conducted on a1024×768 pixels screen with a P4 3.2 GHz, Windows XP.

Task and Stimuli

In the task, participants were asked to control a red cursor movingvertically in a gray rectangular menu. The cursor starts at the top ofthe gray menu when the pressure value is 0. The cursor moves down whenparticipants press the pressure button and moves up when participantsrelease the pressure button. The menu is divided into small units basedon the selected function and the number of pressure levels. The systemrandomly highlights, in yellow, a menu item the user is required toselect. In each trial, participants are required to move the red cursorinto the yellow target area and select the target with a Dwell or Clickselection mechanism, which have been shown to be the best selectionmechanisms for a pressure mouse [24]. The trial ends when theparticipant selects the appropriate target. If the selected item is notthe right target, then the item changes to a dark gray, and the trialcontinues until the participant selects the right target. To selectusing Dwell, users maintain the cursor within the target for 750 ms,whereas Click users click with the left mouse button.

The study used a 4×4×5×2 within-participants factorial de-sign. Thefactors were:

-   -   Function: FE, VF, L, and KC.    -   Relative Pressure Distance: 128, 256, 384, and 512.    -   Pressure Level: 6, 8, 10, 12, and 16.    -   Selection Mechanism: Dwell and Click.

The order of presentation was first controlled for function type, andthen for pressure level. Levels of the other two factors were presentedrandomly. After explaining the selection mechanisms, participants weregiven ample time to practice the techniques. The experiment consisted ofthree blocks with two repetitions per block, per condition.

With 14 participants (9 male, 5 female; average age=27 years), 4functions, 4 distances, 5 pressure levels, 2 selection mechanisms, 3blocks, and 2 trials, the system recorded a total of 13440 trials andthe experiment took approximately 1 hour per user. None of theparticipants had any experience with pressure-based input.

Results

We used the univariate ANOVA and Tamhane post-hoc pair-wise comparisons(unequal variances) for our analyses.

Movement Time:

We found no significant difference between functions (F(3,39)=1.898,p=0.15). FE was the fastest followed by KC, L and VF (see FIG. 13( a).As previously reported [23,28] we too found that Click was significantlyfaster than Dwell (F(1,13)=19.105, p<0.001).

Errors:

Overall, we found a significant difference in E between the functions(F(3,39)=4.264, p<0.01) and selection techniques (F(1,13)=46.91,p<0.001). Post-hoc pair-wise comparison of the functions showed that FEhad significantly fewest errors followed by L, KC, and VF (nosignificant difference between L and KC). FIG. 13( b) shows the averageE for each function. There was a significant inter-action betweenselection technique and function (F(3,39)=7.654, p<0.001). In the caseof Click, the ranking of the functions was similar to that reportedabove, while for Dwell the order was L, KC, FE, and VF. However we couldnot find any significant difference between the functions for Dwellselection technique.

Number of Crossings:

We found a significant difference in NC between the functions(F(3,39)=19.606, p<0.001) and selection techniques (F(1,13)=7.77,p<0.02). Post-hoc comparison of the functions showed that FE hadsignificantly fewer crossings than all other functions followed by KC,VF, and L (no significant difference between VF and L). FIG. 13( c)shows the average NC for each function. We found no significantinteraction between selection technique and function (F(3, 39)=2.86,p>0.05).

Subjective Feedback:

FE was most preferred by nine users followed by L with three, KC withone and one with VF.

Discussion

The results of our experiment reveal that a fisheye function allowsusers to select pressure levels with greater accuracy and with lowernumbers of crossings without losing out on performance time.

Fisheye Improves Pressure Control Across Levels

In line with our expectation, our results show that the method ofdiscretizing the pressure space has a significant effect on the user'sability to control pressure. Additionally, users preferred this functionover all the others. This effect is felt across all pressure levels (seeFIG. 14). We found that the PressureFish function is effective as itprimarily reduces the amount of inadvertent crossings and allows theuser to “lock” into a specific pressure level.3

Results on speed, number of crossings, and accuracy, indicate thatperformance decreases gradually as the number of pressure levelsincreases beyond 6 (see FIG. 14, KC and VF always perform no differentfrom L and have been re-moved from the figure to avoid visual clutter).However, beyond 12 pressure levels, we observe a very sudden drop inperformance with all functions except the fisheye. In the case of thefisheye function, users can control up to 16 pressure levels almost ascomfortably as 12.

Effect of Fisheye Control on Selection Techniques

As reported in previous studies [24, 29] we too observed a larger numberof errors with the Click selection technique in comparison to Dwell. Onereason for this is that any force applied by one finger co-activatesadjacent fingers simultaneously [30]. This effect is pronounced in thecase of the Click selection technique as clicking the mouse button withthe index finger activates muscles in the thumb, which in turninterferes with pressure control on the sensor. However, our resultsshow that the fisheye function operates equally well with both selectiontechniques in terms of error rate, as well as the number of crossings.This suggests that fisheye functions can be universally applied acrossdifferent selection mechanisms. Although untested, we believe thisresult is valid for a pressure sensitive stylus.

Effect of VF and KC on Pressure Control

Our results showed that users had difficulties controlling pressure inthe VF condition. This result is consistent with other similar findingson desktop pointing and focus targeting with Fisheyes which suggest thatdistorting the control space results in better control [23] anddistorting the visual space causes targeting problems if carefulconsideration is not given for the control space [26].

In all conditions, we found no significant difference in performancebetween KC and L. However, in most cases KC was marginally better thanL. This can be attributed to the overlapping pressure units and thecontext-sensitive manner of deciding the pressure level. However, webelieve that better segmentation of the pressure units could be achievedby careful analysis of the different types of errors (over-shoot vs.undershoot) that users commit.

Recommendations to Designers

We provide the following recommendations:

-   -   Designers should consider using a fisheye function to improve        pressure control.    -   A fisheye function will enable the use of a larger number of        discrete pressure levels.    -   Visual feedback is an essential but not sufficient factor for        the enhancement of pressure control.

In this paper we report on the design and effectiveness of PressureFish,a fisheye discretization function that allows users to control pressureinput with fewer errors than previously reported discretizationtechniques, without time penalties and with higher user preference. Webelieve our results will facilitate integrating pressure-based inputwith other input mechanisms. In the future, we will investigate thepossibility of designing pressure menus that behave similarly and thatshare the common advantages of marking-menus. We will also investigateother fisheye functions to improve accuracy and facilitate the design ofnovel and improved navigation techniques such as pressure-scrolling,panning and zooming. Effective control of pressure input can also leadto designs that allow users to manipulate the control-to-display ratioin instances such as cursor control in multi-display or large displayenvironments.

Input devices such as the mouse have witnessed an impressive number ofaugmentations with additional input channels, including extra controlbuttons, the mouse wheel, and sensors. The augmentation of additionaldegrees-of-freedom is motivated by the need to enhance the interactivityof specific tasks at the interface: control buttons for gamingapplications, the mouse wheel for scrolling and zooming, and sensors forswitching between active screens.

In line with recent incarnations of the mouse, Cechanowicz et al [32]have augmented the mouse with additional pressure input channels, andcalled this augmentation the PressureMouse. The PressureMouse buildsupon the recently published set of guidelines for pressure basedinteraction [40,44,46]. However, recent studies on pressure interactionsprimarily provide insight on the strengths and limitations ofpressure-based input and offer guidelines for creating pressureaugmented interactions. Very little is known on how to fluidly integratepressure input channels with the basic operations of the input device towhich it is being augmented.

A large number of pressure based interaction techniques proposed for themouse are based on users manipulating the pressure channel independentlyof the movement degrees-of-freedom [40,44,46].

Related Work

We review the related research on pressure input and integral inputchannels.

Pressure Based Interaction

Numerous studies have proposed novel interaction techniques and haveoffered guidelines for working with pressure based input.

Ramos et al. [46] explored the design space of pressure-basedinteraction with styluses. They proposed a set of pressure widgets thatoperate based on the users' ability to effectively control a discreteset of pressure values. Ramos et al. [46] identified that adequatecontrol of pressure values is tightly coupled to a fixed number ofdiscrete pressure levels (six maximum levels), the type of selectionmechanism and a high degree of visual feedback. However, theirinvestigation does not explore the benefits of simultaneouslyintegrating pressure control with stylus movement.

Mizobuchi at al. [40] conducted a study to investigate how accuratelypeople control pressure exerted on a pen-based device. Their resultsshow that continuous visual feedback is better than discrete visualfeedback, users can better control forces that are smaller than 3N, and5 to 7 levels of pressure are appropriate for accurate discriminationand control of input values. Their results apply to pen based pressureand they do not investigate multi-channel input.

Since controlling pressure input is challenging, Shi et al [47] recentlyproposed PressureFish, a technique to discretize the pressure spaceusing fisheye functions. With PressureFish, users are capable ofmanipulating pressure input with a higher level of control and moreefficiently than common discretization functions.

Researchers studied pressure input in the context of multi-levelinteraction. Zeleznik et al. [49] proposed an additional “pop-through”state to the mechanical operation of the mouse button. As a result, anumber of techniques can take advantage of a soft-press and a hard-presson a pop-through button. Forlines et al. [33] proposed an intermediary“glimpse” state to facilitate various editing tasks. With glimpse userscan preview the effects of their editing without executing any commands.Multi-level input can facilitate navigation, editing or selection tasksbut utilize pressure input in a limited way. In particular, suchtechniques make it further challenging to fluidly control another inputchannel such as mouse movement.

Cechanowicz et al [32] investigated the possibility of facilitatingpressure-based input by augmenting a mouse with either one or twopressure sensors. Such an augmentation allows users to control a largenumber of input modes with minimal displacements of the mouse.Cechanowicz et al [32] developed several pressure mode selectionmechanisms and showed that with two pressure sensors users can controlover 64 discrete pressure modes. Their results also show that activatingpressure sensors that are located near the mouse buttons or located forthumb input are optimal placements for facilitating pressure input.However, Cechanowicz et al [32] did not investigate the possibility offluidly integrating pressure input with other mouse based operations.

While previous studies have guided designers in building systems withpressure input, few results suggest how we can fully integrate pressurewith the underlying input mechanisms of the device to which it isaugmented. Ramos et al [44] proposed Zliding to control a scaling factorwith pressure at the stylus' tip and manipulating a parameter with thestylus' x-y position. Similarly, with Pressure Marks [45] the user caninvoke several states by steering the stylus and simultaneously applyinga high or low pressure value. While both these studies highlight thepossibility of integrating pressure input with the movement of thedevice, they have not explored the large design space that results whenintegrating both input channels. Furthermore, each technique falls shortin inspecting the full range of the input channel: PressureMarks relieson a low or high pressure input (instead of the entire pressure range),and Zliding works within a limited displacement range. Furthermore, verylittle support was provided to users for facilitating simultaneouscontrol more than one non-competing interactive tasks.

In general, very few of the reported results have explored the designspace of fluidly integrating pressure input with the functional featuresof the device being integrated with. Furthermore, little is known abouthow pressure integrates with the very common task of moving a pointer.Based on this limited knowledge it is challenging to proposeapplications that can benefit from integrating pressure with multipleinput channels.

We present PressureMove a pressure based interaction technique thatenables simultaneous control of pressure input and mouse movement.Simultaneous control of pressure and mouse movement can support tasksthat require control of multiple parameters, like rotation andtranslation of an object, or pan-and-zoom. We implemented fourvariations of PressureMove techniques for a 2D position and orientationmatching task where pressure manipulations mapped to object orientationand mouse movement to object translation. The Naive technique mapped rawpressure-sensor values to the object rotation; the Rate-based techniquemapped discrete pressure values to speed of rotation and Hierarchicaland Hybrid techniques that use a two-step approach to controlorientation using pressure. In user study that compared the fourtechniques with the default mouse-only technique we found thatRate-based Pressure-Move was the fastest technique with the least numberof crossings and as preferred as the default mouse in terms ofuser-preference. We discuss the implications of our user study andpresent several design guidelines.

Pressure augmentation could potentially be designed such that the usercan manipulate both pressure input and cursor movement, enabling usersto synchronously perform actions that can otherwise only be accomplishedsequentially. For example, a pressure augmented mouse could potentiallyenable users to rotate and translate an object synchronously, a taskthat is routinely carried out in drawing applications (FIG. 15).

Based on results of an early pilot study and prior work (Zliding [44]and PressureMarks [45]), we observed that users can simultaneouslycontrol pressure and movement, but not all users utilize thesimultaneous control in fluid fashion. In this paper we investigate thedesign space and the resulting interaction techniques that allowsimultaneous control of pressure and movement, referred to asPressureAlove. To demonstrate the effectiveness of PressureMove, weconcentrated on the task of simultaneous rotation and objecttranslation. We designed four PressureMove techniques that provide usersthe flexibility of using the input dimensions of pressure and movementsimultaneously or sequentially.

Pressure manipulations controlled object orientation and mouse movementcontrolled movement. The first a Naive technique mapped the raw pressurevalues from the sensor to the rotation of the object while mousemovement mapped to object translation. The second technique, referred toas PressureMove Rate-based, was inspired by tap-and-refine [32] andmapped the rate of pressure change to rotation angle. The thirdtechnique is an Hierarchical technique that uses discrete pressurelevels for object rotation in two steps—a coarse grain and a fine grainstep. Finally, we included a Hybrid technique that combined thesimplicity of Native technique with the multi-step control ofHierarchical. In a 2D rotate and translate task, similar to thetetrahedral docking task in 3D [38,51], we examined the proposed designsfor integrating mouse movement and pressure rotation. Our results showthat the Rate-based integration offered best control and performance.The Rate-based technique was significantly faster than all othertechniques including the traditional mouse. The Naive implementation wasas fast as the conventional mouse in terms of trial completion times butwas significantly slower than the traditional Mouse and Rate-basedtechnique in terms of crossings.

The main contributions of this paper are to: 1) extend the design spaceof a pressure augmented device (the mouse) to include simultaneouscontrol of pressure and movement; 2) design integral interactiontechniques; 3) identify strengths of various strategies for controllingnon-competing degrees-of-freedom; and 4) outline design implicationsthat emerge from our systems.

Fluidly Controlling Multiple Input Channels

There has been a long standing interest in identifying how to integrateand facilitate control of simultaneous input channels. Jacob et al [43]proposed a framework that can facilitate the understanding andcategorization of integrality and separability of input devices andinteractions afforded by these. Two input dimensions are consideredintegral if they are perceived as a single dimension or separable if thedimensions seem unrelated [43]. In their study, performance was betterwhen the device matched the tasks in integrality/separabilitydimensions. In light of their findings, coordinating multiple channelsmay suggest whether the input device is operating in the same dimensionspace as the task, i.e. good coordination and performance suggests thatthe device and perceptual structure of the task are in the same space.Integrality can be considered to some ex-tent as a coordination measure.

Balakrishnan et al [31] used integrality to demonstrate that subjectscould control three degrees of freedom simultaneously with the Rockin'Mouse, a X-Y translational and one Z-rotational DOF. Similarly,MacKenzie et al. [37] investigated the possibility of integratingrotation on the mouse, a device designed primarily for translation andselecting objects. The TwoBall mouse facilitates a number of commontasks, and makes certain application features, such as the rotate tool,redundant.

Studies have also investigated the benefits and possibility ofintegrating several tasks into one coherent and fluid action. Kruger etal. [35] designed a technique, RNT (Rotate'N Translate), to fluidlyintegrate rotation and translation. The motivation behind RNT was toprovide in one seamless action the ability to rotate and translate anobject in a collaborative environment. The behavior of RNT simulates thephysical behavior of dragging a sheet of paper on a table. Results oftheir study show that RNT is more efficient than separately controllingtranslation and rotation. RNT further enhances a number of collaborativetasks, including coordination and communication with respect to userorientation.

Fluid integration of multiple input channels was examined in the contextcontrolling an input device with the fingers instead of using the entirearm. In an empirical study, Zhai et al [51] investigated theeffectiveness of finger muscle groups in controlling multipledegrees-of-input. Zhai et al [51] gave users two alternative 6DOF inputdevices, one that controlled a cursor with the movement of the entirearm (glove) and the other with the fingers of a hand (FingerBall[X]).The objective of the study was to assess whether finger control was moreeffective than arm control in finely rotating and positioning an objectin 3D. The task consisted of docking a cursor with the target, both ofwhich were equal size tetrahedral. They found that the finger-baseddevice facilitated better control and afforded simultaneous translationand rotation actions.

In developing a metric for measuring the allocation of control in a 6degree-of-freedom rotation and translation task, Masliah and Milgram[38] studied the interdependence and overlapping actions of the twotasks. They used a 3D virtual docking task, similar to that of Zhai [50]in which subjects were asked to align a tetrahedral shaped cursor ontoan identically shaped target. Interestingly, their results showed thatusers would rarely control all 6 DOFs simultaneously. Instead, userswould allocate their control to the rotational and translational DOFsseparately. However, with practice they found that allocation of controlwithin the translational and rotational components of the taskincreased.

Wang et al [48] carried out a study to investigate the relationshipbetween object transportation and object orientation by the human hand.In their experiment, subjects were asked to align a small wooden blockwith a graphical target cube. Manipulation tasks were designed thatrequired both object translation and orientation, under different visualfeedback conditions. Their results demonstrate the existence of aparallel and independent structure for object translation andorientation which is persistent over different visual feedbackconditions. Their results suggest that object translation andorientation seem to share characteristics of an integral structureaccording to the notion by Jacob et al [43].

PressureMove

We propose PressureMove, a pressure based technique that facilitatessimultaneous control of mouse movement and pressure input. PressureMovemaps mouse displacement onto object movement and pressure input ontoobject rotation. In designing PressureMove we needed to consider twoprimary dimensions: controlling pressure input, and visual feedback.

Controlling Pressure

Sensors typically report pressure values between 0 to 1024 levels.Previous studies have suggested that users are not capable ofdistinguishing the granularity and controlling this range of pressurevalues [32,40,46]. This has led most investigations to discretizing thepressure space into controllable and haptically perceivable units. Ramoset al. [44,46] revealed that adequate control of pressure values istightly coupled to a fixed number of discrete pressure levels (maximumof six levels). Cechanowicz et al. [32] suggested that pressurediscretization can include 8 to 10 discrete levels, when controlled bythe thumb or index finger, on a mouse.

Furthermore there are no standard mechanisms to discretize the number ofpressure levels obtained from the sensors. There are many methods andmappings for discretizing the number of controllable levels using apressure-based de-vice. These include: a linear discretization function[40,46]; a parabolic-sigmoid discretization function that results in aslow response at low pressure levels, linear behaviour in the middlelevels, and a slow response at the high levels of the pressure range[44]; a quadratic discretization function [32] that allocates largerpressure ranges at the lower levels and smaller pressure ranges athigher levels; a fisheye function that provides a larger space aroundthe position of interest in the pressure space [47].

An alternative to discretizing pressure input is to map the raw pressurespace (non-discretized—referring to the fact that the discrete pressurevalues reported by the sensor are not further discretized) onto the taskparameters. Each unit of pressure in the raw pressure space controls aninput parameter, whether it be angular rotation, scalar, or otherfactor. Raw pressure input is not easily controlled, however facilitatesa larger number of mappings.

We can also define a hybrid pressure space that is composed ofcontinuous and discrete pressure values. With hybrid control, continuouspressure input can provide the user with rapid access to a region ofinterest within the pressure space while switching to discrete controlallows finer granularity and control over parameter values.

The design of PressureMove includes discrete, raw, and hybrid pressurecontrol techniques.

Visual Feedback

Kinesthetic feedback alone is insufficient for adequately controllingpressure. Visual feedback is a dominant characteristic of mostclosed-loop pressure based interactions [32,40,44,46]. Different formsof Visual feedback for pressure based input have been explored inPressureWidgets [46]. However, the Visual feedback in PressureMove isinspired by the visual feedback mechanism used by Kittenakare et al [34]and Ramos et al [46]. Since the design of the visual feedback isintricately tied to the task, we de-scribe the feedback designed for thetask of simultaneously positioning and orienting an object. We expectthat a similar form of visual feedback can be easily adapted for othersimultaneous control tasks.

A pressure cursor is used to provide appropriate visual feedback. Thedefault cursor is a solid triangular shaped object (see FIG. 16( a)).When the user applies pressure a proportion of this cursor getshighlighted relative to the amount of pressure being applied as in FIGS.16( b) and 16(c). Visual feedback is always continuous, as this form offeedback has shown to enhance performance over non-continuous visualfeedback. Additionally, we redundantly encode pressure amount to theaperture of the pressure cursor, i.e. the higher the pressure value, thelarge the aperture of the cursor (as is seen in the difference in sizeof the cursor in FIGS. 16( a) and 16(b)).

In the case where we used a hybrid pressure space we used a two-stepcursor as shown in FIGS. 16( d) and 16(e). The head-triangle (thetriangle that represents the head of the cursor) represents the firstpressure space the user can use while the second triangle corresponds tothe second pressure space. In FIG. 16( d) the user is currentlycontrolling the first pressure space while in FIG. 16( e) the user isoperating with the second pressure space. In cases where multiplepressure spaces are composed to form the technique, multiple trianglescan be concatenated. However, in our design we only used up to twopressure spaces composed to form a single technique.

PressureMove Techniques

We describe four variations of PressureMove techniques that can becreated to manipulate mouse movement and pressure input simultaneously.All pressure interaction techniques used the thumb sensor to manipulatethe parameter in one direction and the middle finger sensor tomanipulate the parameter in the reverse direction.

PressureMove—Naive

As the name suggests this is a naive implementation of a simultaneouscontrol technique. In this technique the raw pressure values reported bythe pressure sensor are mapped to the object parameter controlled bypressure. FIG. 17( a) shows the mapping function—the pressure range ismapped to the complete range of the rotation parameter, i.e. 360° angle.When the user increases pressure the object orientation increases andwhen they release pressure the orientation reverses i.e., if the initialdirection of rotation is clockwise then on releasing pressure the objectchange orientation in the counter-clockwise direction. When the userreleases the pressure sensor the parameter value returns to the startingposition. To fix the value the user can left-click before releasingpressure. When the user presses the thumb sensor the object rotatesclock-wise and the visual feedback is as shown in FIG. 16( b). When theuser switches to the sensor located on the middle finger the objectrotates counter-clockwise.

PressureMove—Rate-based

In this technique each level of the discrete pressure space maps to thespeed of rotation of the object as shown in FIG. 17( b). When the usermaintains pressure at discrete level 1 the object rotates by 1° at eachtimer event. To move the object faster the user moves higher up withinthe pressure levels. At level n the object rotates at n degrees pertimer event. This mechanism provides the additional benefit ofmaintaining a given orientation when the user releases the pressuresensor, thus incorporating a clutching mechanism that is not availablewith the naive technique. At discrete level 0 the user can tap thepressure sensor to nudge the object by I° per tap. This gives the useradditional fine control when honing in on the target. This tapping wasinspired from the Tap-and-Refine technique in [32]. The visual feedbackused was the same as for the Naive implementation.

PressureMove—Hierarchical

PressureMove-Hierarchical allows users to control rotation in twosteps—a coarse-step and a fine-step. The coarse and fine movement iscontrolled by a discrete pressure map-ping. In the coarse-step moving toa pressure level 1 results in rotating the object by 24° (one step is360°/15 levels=24°) and moving up successive levels rotated the objectby 24° per level n (n C [0,15], n is the coarse-step pressure level).Thus at any pressure level the object is rotated by n*24°: while in thefine-step moving up each pressure level rotates the object by 1°starting from n. The object rotates from n to n*24−15 using one sensorand from n up to n*24+15 using the other sensor where n is the point inthe coarse-control when the user switches to fine-control. The user cantoggle between coarse- and fine-step by using the left click button.FIG. 17( c) shows the pressure vs angle profile for this technique. Thedotted line at about 150° indicates the moment at which the user movedfrom coarse to fine control using left-click. FIGS. 16( d) and 2(e) showthe visual feedback that was provided to the user when using the thumbsensor (so object rotates clockwise). The top triangle of the cursorchanges with pressure when the user is performing a coarse-level action(as in FIG. 16( d)) and the bottom triangle changes with pressure whenthe user if performing a fine-level action (as in FIG. 16( e)).

PressureMove—Hybrid

Hybrid combines the simplicity available with Naive with the finecontrol provided by Hierarchical. The coarse-step of Hierarchical isreplaced by the continuous rotation control used in Naive (see thebottom left part of FIG. 17( d)). This enables the user to quicklyrotate the object to approximately the desired orientation and (b) thenuse finer step control to perform a more precise orientation. Thefine-control step and the visual feedback mechanism worked exactly as inHierarchical.

Experiment

The goal of this experiment was to evaluate PressureMove as a viableconcept for simultaneous control of pressure input and mouse movement.We specifically evaluated the four PressureMove techniques, to assesstheir strengths and weaknesses, with a canonical task.

Task and Stimuli

The task, shown in FIG. 18, required the user to reposition and reorientto a target location and orientation a small object (100×100 pixels)which initially appeared upright and in the left end of the screen. Thetarget, of a slightly larger size than the object appeared to the rightof the object. The size, the distance to the object and the orientationof the target were changed as part of the experimental design.

Users could see the object and the target before the beginning of eachtrial. The trial began when the user moved the cursor onto the objectand pressed the left mouse-click. The user repositioned and reorientedthe object to the target location using the different interactiontechniques. When the object position and orientation matched the targetposition and orientation, the target bounding rectangle changed to agreen color. The user then had to maintain the matching position andorientation for 1 second before the trial was completed. We did this toprevent users from accidentally matching the position and orientation.If the user moved the object away from the matched position, the Isecond timer was reset. The object position and orientation wereconsidered to match those of the target if the difference in pixels andorientation was within the target-fit parameter controlled as factor ofthe experiment. When the trial is completed the target boundingrectangle briefly turns red and the next trial loads.

Hardware Configuration and Techniques

Our study used an optical mouse with pressure sensors mounted on its rim(FIG. 15). The sensors (model #IESFR-5L from CUI Inc.) could measure amaximum pressure value of 1.5 Ns. Each sensor provided 1024 pressurelevels. The application was developed in C# and the sensor wascontrolled using the Phidgets library [41]. The experiments wereconducted in full-screen mode at 1280×800 pixels on a Intel T5600 1.83GHz, Windows Vista OS. Two sensors were mounted on the mouse such thatthey could be easily accessed by the thumb or the middle finger (asshown in FIG. 15). All pressure interaction techniques used the thumbsensor to rotate the object clockwise and the middle finger sensor torotate the object counter clockwise.

For all the discrete pressure based techniques we used the PressureFishdiscretization function [X] with 15 pressure levels. For the continuouspressure cases we only used pressure values between 0 and 720 asprevious research has shown that users find it difficult to maintainpressures at higher values.

Procedure and Design (115 page)

The study used a 5×2×3×2 within-participants factorial design. Thefactors were:

Technique: Naive, Rate-based, Hierarchical, Hybrid, Mouse-only.Distance: 500 pixels, 1100 pixels.

Orientation: 60, 135, 270.

Target Fit: tight, loose.

The order of presentation first controlled for technique and then fordistance followed by orientation and target-fit. A tight target-fitmeant that the users had to position the center of the object within ±4pixels of the target center and the object orientation has to be within±5 degrees of the target orientation. In the case of loose target-fitthese figures were ±12 pixels and ±8 degrees respectively. We explainedthe techniques and participants were given ample time to practice thetechniques at the beginning of the experiment. The experiment consistedof three blocks with each block comprising of two repetitions for eachcondition. With 5 techniques, 2 distances, 3 orientations, 2target-fits, 3 blocks, and 2 trials, the system recorded a total of(5×2×3×2×3×2) 360 trials per participant. The experiment tookapproximately 60 minutes per participant.

Performance Measure and Participants

The experimental software recorded trial completion time, and number ofcrossings as dependent variables. Trial completion time (MT) is definedas the total time taken for the user to position and orient the objectwithin the target. The number of crossings (NC) is defined as the numberof times the object enters and leaves the target position or orientationfor a particular trial. Users were not able to proceed to the next trialwithout successfully completing the task and so there were no errors forthe software to record. While MT gives us an overall success rate, NCprovides information about the level of control achievable using each ofthe different pressure control mechanisms. Participants were also askedin an exit questionnaire to rank the different pressure controltechniques in terms of mental demand, physical demand, effort, overallperformance and frustration.

Thirteen participants (11 males and 2 females) between the ages of 19and 40 were recruited from a local university. All participants hadprevious experience with graphical interfaces and used the mouse intheir right hand. However, none of the participants had worked with apressure based input device before.

Results

We used the univariate ANOVA test with participant number as a randomfactor and Tamhane post-hoc pair-wise tests (unequal variances) for allour analyses.

Completion Time

The average trial completion time was 6.1 s with a standard deviation of4.9 s. Out of a total of 4680 trials 73 outliers (more than 3.5 standarddeviations from the group mean) were excluded from further analysis.There was a significant effect of interaction technique (F(4,48)=11.15,p<0.001), target-fit (F(1,m=102.9, p<0.001), distance (F(1,12)=7.5,p<0.02), Orientation (F(2,24)=15.9, p<0.001) and block-number(F(2,24)=43.4, p<0.001) on MT. FIG. 19 shows the mean trial completiontime for each technique and target-fit. Overall, Rate-based was thefastest technique followed by Naive, Mouse, Hierarchical, and Hybrid.

Post-hoc analysis showed that all pairs were significantly differentexcept (Naive, Mouse), and (Mouse, Hierarchical). Block 3 wassignificantly faster than Block 2 which was significantly faster thanBlock 1. Users were significantly slower in completing the trials whenthe target-fit was tight (as opposed to loose); when targets werefarther (1100 pixels followed by 500) and when the orientation of targetwas greater (all combinations significantly different with 270 deg>135deg>60 deg).

Crossings

The M R Hy Hi N average number of crossings per trial across allconditions was 2.5 (standard error=0.048). There was a significanteffect of interaction technique (F(4,48)=55.15, p<0.041), target-fitW(1,12)=68.1, p<0.001), distance (F(1,121=7.5, p<0.02), Orientation(F(2,24)=19.8, p<0.001) and block-number (F(2,24)=13.7, p<0.001) on MT.We found no effect of target distance on number of crossings. FIG. 20(a) shows the mean crossings for each technique. Overall Rate-based hadthe least number of crossings, followed by Mouse, Hierarchical, Naiveand Hybrid.

Post-hoc comparisons showed that there was a significant differencebetween all pairs except the (Rate-based, Mouse), (Mouse, Hierarchical)and (Naive, Hybrid). Block 3 had significantly fewer crossings (mean2.1) than Block2 (mean 2.4) which in turn had significantly fewercrossings than Block! (mean 2.9). Users had significantly fewercrossings in the loose target-fit condition (mean 2.1) than in the tighttarget-fit condition (mean 2.9). Users had significantly fewer crossingswhen the target orientation was 270 deg (mean=2) when compared to 60 deg(mean=2.7) or 135 deg (mean=2.9). We found no statistical difference innumber of crossings between 60 and 135 deg.

Subjective Ranking

In terms of overall performance users ranked the Mouse as the besttechnique followed by Rate-based, Hybrid, Hierarchical and Naive. Anovatest on the overall performance revealed a significant difference interms of user ranking between the different techniques (F(4,64)=16.6,p<0.001). Post-hoc analysis did not reveal any significant differencesbetween (Mouse, Rate-based) and (Hierarchical, Hybrid) pairs. But allother pairs were significantly different.

We found similar rank-ordering of the techniques in terms of OverallEffort, Mental Demand, Physical Demand, and Frustration (see FIG. 20(b)). In all cases Mouse was the most preferred technique (leastdemanding and frustrating) closely followed by Rate-based. Theleast-preferred technique was the Naive implementation. In all cases,Anova showed significant difference in user ranking and post-hocanalysis revealed no significant difference between the (Mouse,Rate-based) pair and (Hierarchical, Hybrid) pairs but all other pairswere significantly different.

Discussion

Performance Improved over Blocks

As can be seen in FIG. 21, users were constantly improving theirperformance over the three blocks for both trial completion times (FIG.21( a)) and number of crossings (FIG. 21( b)). The average MT in Block 3was 5.0 s corn-pared to 5.3 s for Block 2 and 6.1 s for Block I.However, the univeriate analysis we used in the previous section did notreveal any significant interaction between technique and block numberfor both MT and Crossings. Thus while users continued to improve theirperformance over each block the overall order of the differenttechniques did not change. Observing improvement over blocks is in linewith prior work that suggests that with practice users are able toallocate better control to the simultaneous operation of different inputdimensions of the task [38].

Effect of Orientation

The Hierarchical technique was more markedly affected by the targetorientation than the other techniques. Especially when the targetorientation was 270 deg, the average MT for Hierarchical was about 7.2 scompared to 5 s for the other two orientations (see FIG. 22( a)). Toselect the target at 270 deg orientation users often used the thumbsensor that rotated the object clockwise. This meant that the user hadto maintain pressure at level 11 while selecting the left-click to go tothe fine level of control. As users found it difficult to maintainpressure steadily at this level, they often lost the level while tryingto click requiring them to click again to comeback to the coarse-level.This resulted in a large increase in the number of clicks when thetarget orientation was 270 deg (see FIG. 22( b)).

However, when the target orientation was 135 deg, users had to maintainpressure at level 5 which was in the middle region of the entirepressure range making it easy for users to maintain pressure at thislevel, resulting in markedly smaller number of clicks and consequentlyfaster trial completion times. From FIG. 22( a) one can see that for thetarget orientation of 135 deg Hierarchical was the fastest techniquewith an average MT of 4.3 s compared to 4.5 s from the next fastesttechnique—Rate-based.

Overall, while we believe we can explain users' performance with thehierarchical technique, we are not sure why users did not use the secondsensor to rotate the object counter-clockwise. Users were made aware ofthis option during training and they often used the second sensor(without hesitation) in the Rate-based and Naive techniques.

Simultaneous Control

As part of the experimental log we collected continuous data of mousemovement and pressure values for each trial. FIG. 23 shows typicalmovement and pressure profiles for the four pressure-based techniques.Each left-right pair is distance and pressure profile for the same trialof a user. However, each technique is from a different user, selectedrandomly to highlight that the movement profiles shown in the figuresare stereotypical. In the left images when the distance is not changingthe user has positioned the object near the target whereas the sameinterpretation is not true for all pressure based techniques. TheRate-based technique being a relative input technique, users don't needto maintain constant pressure to complete the task

We can see from the figure (FIGS. 23( a) and 23(b)) that the Naiveimplementation does not really encourage simultaneous control ofpressure and movement. Users use the first second to completepositioning the object before applying pressure to change orientation.We observe a similar trend with the Hybrid technique.

In the case of the Hierarchical technique, users start applying pressureabout the same time that they start moving (see FIGS. 23( e) and 23(f).But in the first part of their motion (between 0 and 2 s) they mostlyfocus on moving the object to the right location and then switchattention to orienting (between 2 and 4 s) the object.

But in the case of Rate-based technique, users start applying pressureto change orientation at the same time as they are moving the object toposition it. In FIGS. 23( g) and 23(h) we see that in the first 2 s theuser is both positioning the object while at the same time as applyingpressure. However, unlike the Hierarchical technique, they havecompleted most of the positioning and orienting within the first 2 s andbetween 2 and 4 s they are merely fine-tuning the object. We believethat the open-loop motion for both positioning and orienting coincidemaking rate-based technique a powerful PressureMove technique. However,we did not test this hypothesis with our data.

We observed similar profiles across all users and believe thatPresssureMove technique that's based on a rate-based mapping encouragesusers to simultaneously control both movement and pressure.

PressureMove—Rate-based

Our results, across all conditions show that PressureMove—Rate-basedoutperforms all other PressureMove designs. Several unique properties ofthe rate-based technique con-tribute to its superior performance.

First, this technique is based on discrete pressure control. Since eachlevel of the discrete pressure space maps onto the angular speedparameter, a high degree of control is required to hold and maintain thepressure at given discrete levels. This is facilitated by a small numberof discrete pressure levels and by the use of discrete fisheye function.

Additionally, since each pressure level is assigned an angular velocity,pressure level 0 brings the rotating object to a halt, at the lastapplied orientation. The implicit clutching mechanism in the rate-basedtechnique allows smaller close-loop movements than the other techniques.Finally, the technique allows fine adjustments at level 0, by nudgingthe object by I° every tap. The fine grain control over angulardisplacement and the fluidity of this technique facilitates a higherdegree of simultaneous control than any of the other systems.

Results similar to ours which show that rate-based technique improvesperformance in certain types of input devices, have been observed in 3Dpositioning tasks. Zhai [50] points out that using isometric devices(such as a joy-stick that self-centers) to operate in a position controlmode (or zero-order) results in poorer performance than when operatingthe device in a rate-based controller (or a first-order) mode.

Applications

PressureMove can enhance the interactive performance in a number ofdifferent applications. In all of the following applications, thesimultaneous control of more than one input parameter would ease thetask of the operator. While we have not evaluated each of thePressureMove techniques for these applications, we believe theRate-based implementation of PressureMove would offer improvedperformance.

Zoomable User Interfaces

Zoomable user interfaces can largely benefit from the simultaneouscontrol of several parameters. In Zliding, the user was given theability to control scale and the resolution of the scale. In a similarmanner PressureMove can control various parameters by applying pressureto a scalar value and movement to direction of the zooming operation.For instance moving the mouse left or right could zoom in or outrespectively, while pressure would control the resolution factor of thezooming operation. In terms of implementation, the rate-based techniquewould change the resolution of the zoom operation by one step at eachlevel of angular velocity. Similarly, a combined Pan+Zoom interfacecould be easily implemented using PressureMove. For example on a map,the mouse movement would pan the document while pressure input zooms inor out. Seamless and integrated panning and zooming has shown to improveperformance over manipulating each dimension separately. Finally, inmost ZUI implementations the center point of reference in the zoominterface is defined by the position of the cursor or crosshair beforetransitioning into the zoom. However with PressureMove, the position ofthe cursor can be updated dynamically during zoom transitions, therebyfacilitating a larger degree of freedom in moving around a workspacewhile zooming. In all these applications undoing or returning to aprevious state is easily achieved by using the additional pressuresensor.

Drawing Applications

Drawing applications facilitate a large number of object positioningtasks with operations that involve rotating elements, scaling and/orskewing. In these applications, operations requiring coarse orapproximate movements (such as scaling or skewing an object) could berelegated to the pressure input and precise positioning could beassigned to the mouse movement. Two pressure sensors would be requiredto undo operations, a feature that is currently part of PressureMove.Furthermore, we PressureMove can be adapted for object manipulation andpositioning in 3D. CAD systems could utilize the pressure inputdimension to rapidly rotate the entire scene, thereby making accessiblea different aspect of the 3D drawing.

Pressure Menus

While in systems such as PressureMark where users controlled a high orlow pressure value, our results demonstrate that with PressureMove userscan control several intermediary pressure levels during movement. Thiscan be particularly useful for designing (as was done withPressure-Marks) an interactive menu in which different menu items aretriggered based on the pressure level invoked during movement. ThusPressureMove could integrate fluid menu invocation with object selectionas is done with techniques such as zone and polygon[52] or marking[G]menus or others similar techniques developed for styluses.

Dynamic Control-Gain

PressureMove could be utilized to dynamically manipulate control gainratios. Such manipulation is particularly useful on high resolution,large display interactions on which users operate with fine and coarseresolution. While the applications suggested earlier utilize pressureinput for a task independent of cursor movement, in this applicationusers would be controlling one task dimension with two input channels.We believe that novel design spaces and solutions can result frominvestigating the use of PressureMove in such environments.

Design Recommendations

There are several lessons that designers can take from ourinvestigation:

Pressure input can be appropriately integrated with mouse movement, suchthat both dimensions are operated simultaneously. This should result inhigher performance gains than operating with either channel separately.

PressureMove—Rate-based should be the first and preferred implementationof any PressureMove application. The discrete pressure control, finegrain pressure mapping and inherent clutching mechanisms in therate-based techniques are favorable properties that could be borrowed toimplement other variations of PressureMove for simultaneous pressure andmovement control.

Allowing users to gain experience with PressureMove is important and maybe necessary in some cases, i.e. any new implementation of aPressureMove technique should not be discarded without first givingconsideration to proper training.

PressureMove is a novel technique that facilitates the simultaneouscontrol of various input parameters. We de-signed PressureMove tospecifically facilitate object rotation with pressure input and objectmovement with the mouse displacement. We designed and implemented fourPressureMove techniques, based on existing pressure-based interactions[32,44]. Our PressureMove techniques cover the wide spectrum ofpossibilities with pressure control and mouse displacement mappings. Ina study, the Rate-based PressureMove technique, which maps pressureinput to angular velocity allowed the maximum amount of simultaneouscontrol of pressure with mouse movement. Users were able to perform adocking task more efficiently and with fewer crossing with therate-based implementation. We have demonstrated the possibility ofsimultaneous control of pressure input and mouse movement. We believeother similar interactions involving simultaneous pressure and movementare possible and will enhance the interactive performance on tasks withmultiple input dimensions.

Pressure input has become a topic of significant interest within the HCIcommunity as researchers slowly gain insight on how to harness thepotential of pressure based interaction. Researchers are devising newpressure-based input devices [56,79], integrating pressure input intoexisting devices [59,61] or are exploring the limitations to pressureinput [70, 73, 76-78]. To make this form of technology more widespreadand applicable in common interactive tasks, researchers are producing afair amount of knowledge on some of the key aspects of pressure basedinput such as identifying the number of pressure levels that are easilycontrollable, the necessity for visual or haptic feed-back, or thelimitations to controlling pressure in a bidirectional manner. Despitethe significant progress in this area over the past few years, aquestion that lingers concerns how to bring these pressure sensinginteractions closer to the average user's daily interactive activities.

Commercially, several input devices facilitate pressure basedinteractions, such as with a stylus on a tablet PC. However, pressuresensing interactions are not limited to pen-based systems. Recently,Apple introduced the MightyMouse™ [72] that is equipped with twopressure sensing buttons attached to the opposite sides of the outer rimof the mouse. Integrating pressure buttons on a mouse as that employedby the MightyMouse™ is analogous to adding additional buttons to micefor managing multiple windows, for scrolling documents or for enhancinggaming activities. However, such enhancements can provide very limitedinteraction bandwidths as it can be difficult for a user to benefit fromthe different buttons due to the ergonomics, the physical spacelimitations of the mouse and the potential conflict that may arise fromplacing the buttons in inappropriate locations on the mouse.

Ideally, it has been suggested that pressure sensing capabilities shouldbe added to a device without significantly changing a device's formfactor [79]. A potentially compelling design that can seamlesslyintegrate pressure interaction onto the mouse would consist ofsubstituting or augmenting the current primary left and right mousebuttons with pressure buttons as in FIG. 24. If such a design were to besuccessful, mouse designers could give users the freedom to select(click) and trigger actions (double-click) as is currently carried outwith mouse buttons in addition to facilitating continuous pressurecontrol at the interface. This approach is similar to that proposed byZeleznik et al [83] in the pop-through mouse. However the pop-throughmouse only facilitates interaction with three discrete states (softclick, hard click, release) and as a result constitutes a limited formof pressure-based input.

To facilitate an interchangeability of pressure sensors with mousebuttons, at a minimum two primary and fundamental features of a regularmouse button would need to be replicated: selection and actioninvocation. Selection is commonly performed by clicking while triggeringan action (such as opening a file or application) is handled bydouble-clicking on the mouse button.

Related Literature

We review the literature in three related areas: pressure-basedinteraction, input selection techniques and pressure-based selectiontechniques.

Pressure Based Interaction

Numerous studies have demonstrated the effectiveness of, and haveoffered guidelines for working with pressure based input.

A number of studies have investigated the design space resulting frompressure based interaction with styluses [70,76,78]. Pressure input witha stylus is captured by directly applying a force on the tip of thestylus and orthogonal to the surface of a screen. In one study, Ramos etal. [76] approached the design space by identifying the con-fines ofinteracting with pressure widgets. Mizobuchi et al. [73] were interestedin studying the degree of accuracy that is achievable by controllingpressure with a pen-based device. The results of these investigationsshows that pressure control is most effective when the pressure space isdivided into approximately 6±1 discrete pressure levels, when pressurecontrol is kept under 3N and when a high degree of visual feedbackaccompanies pressure based input [70,73,78,82]. Their results areprimarily applicable to the use of pressure based interaction with astylus and they did not investigate the implications of interchangingprimary selection mechanisms on a stylus with pressure sensors.

In a recent study Ramos et al [77] designed pressure marks, a fluidpressure-based input with pen strokes to combine selections and actionsat the interface. Pressure marks are designed in such a manner thatusers can make a stroke with varying levels of pressure to trigger anaction. In a study, pressure marks which allow users to specifyselection and action concurrently outperformed existing techniques thatrequire these operations to be performed in a sequential manner.

For over a decade, isometric devices have used pressure input as amethod of controlling the user's mouse cursor. In such systems usersdecrease or increase the amount of force on an isometric pointing nub tocontrol the velocity of the cursor. The PalmMouse™ [74] integratesisometric control into a mouse by allowing users to control cursor speedby applying a slight amount of pressure to a navigation dome which isplaced on the top of the mouse. Isometric devices map pressure input tothe speed of the cursor and have not been designed for substituting theselection mechanisms of buttons on a mouse.

Touchpads that sense pressure are widespread input devices in notebooksor portable music players. Researchers have successfully integrateddiscrete mechanisms of selection and action with continuous pressurebased input with touchpads [79]. On a touchpad, users can perform asingle tap or double tap to trigger a selection or an action,respectively. Additionally, with a touchpad, continuous pressure inputis used to for mapping various functions, such as scrolling. Pressuresensing is utilized in a limited manner on touchpad based input throughwhich a user can control the document scrolling rate by pressing ontothe edge of the touchpad.

One recent development that has largely motivated the re-searchpresented in this paper was the development of a pressure augment mouse[59]. Cechanowicz et al [59] investigated the possibility offacilitating pressure-based input by augmenting a mouse with either oneor two pressure sensors. Such an augmentation allows users to control alarge number of input modes with minimal displacements of the mouse.Cechanowicz et al [59] developed several pressure mode selectionmechanisms and showed that with two pressure sensors users can controlover 64 discrete pressure modes. Their results also show that activatingpressure sensors that are located near the mouse buttons or located forthumb input are optimal placements for facilitating pressure input.However, Cechanowicz et al did not investigate the possibility offacilitating all selection-based operations on pressure-augmented mousesuch as the mouse click and double-click.

Several other researchers have shown the potential of extending thepressure sensing capabilities of touchpads to provide for a richer setof interactive capabilities. Blasko and Feiner [56] proposed multiplepressure-sensitive strips by segmenting a touchpad into differentregions. They show that pressure-sensitive strips do not require visualfeedback and users can control a large number of widgets using theirfingers. Rekimoto and Schwesig [79] propose a touchpad-based pressuresensing device called PreSensell that recognizes position, contact areaand pressure of a user's finger. PreSensell eliminates the need forvisual feedback by providing an amount of tactile feedback proportionalto the amount of pressure being applied onto the touchpad.

The left mouse button serves a vital purpose in a GUI as it enablesusers to select an object through a single or a double-click. To invokea basic button click the user applies sufficient pressure on the buttonbeyond a fixed threshold. Users get both aural and haptic feedbackduring the clicking process. However with a mouse button user input isrestricted to a single or double-click. Here we investigate the benefitsand trade-offs of using pressure sensors for a mouse left-button. Whilesensors lack any form of haptic or aural feedback mechanism, they areeffective in allowing users to control a continuous range of pressurevalues thereby facilitating a wide range of input. In a first study wecompared the time it takes the user to click with a pressure sensor incomparison to the mouse button. Our results show that users can click aseffectively with a pressure button as with a regular mouse button. In afollow-up study we compared two pressure sensor based double-clickstrategies to the traditional button double-click. We found thatpressing on pressure sensors is an excellent substitute to mouse doubleclicks. Overall, our results suggest that buttons can be effectivelyreplaced by pressure sensors for actions that involve clicking anddouble clicking.

In this paper we introduce and evaluate several potential designs for amouse click with pressure sensors. Results of the first study show thata single click can be effectively replaced by a pressure click. Based onthe results of the first study we design several other pressure clickingmechanisms to replace the mouse double-click. The results of a secondstudy show that a hard press with a pressure sensor is more effectivethan a double click. The results overall open up the potential ofenhancing mouse buttons with pressure sensors so that a wider range ofinput modes can be accessed with one of the most commonly used inputdevices.

The main contributions of this paper are to: 1) extend the potential ofa mouse with pressure sensing input; 2) identify strategies for invokingmouse clicks with pressure sensors; 3) identify possible design elementsfor replacing current clicking mechanisms.

The idea of interacting using either continuous pressure modes ordiscrete selection action modes with one pressure sensing inputmechanism, similar to that on touchpads has inspired to a certain extentthe development of the systems presented here.

Input Selection Techniques

Pointing and selecting objects is considered to be a primary andnecessary operation for most common forms of interactions. If weconsider pointing and selecting as two separate processes, we can referto pointing as the movement of a cursor starting at some initialposition and ending on the target, and selection as the initiation of abutton click and release. Some evidence suggests that selection alone(i.e. button clicking) without pointing can consume a significant amountof the total target selection time [57,68]. As a result, enhancing theselection mechanisms on an input device can lead to more efficientinteractions. Most commonly available input devices such as the mouse,the stylus or touch-screens have witnessed several enhancements forreplacing or improving selection.

On a mouse, selection is achieved by clicking on one of two or threeprimary buttons. Designers have proposed several alternatives tobutton-clicking. Bohan and Chaparro [57] compared a mouse-click to adwell-to-click, or hover. In their study Bohan and Chaparro found that ahover of 200 ms provided a gain as high as 25% for task completion timesin comparison to a mouse button press and release [57]. The GentleMouse™[64] is a commercial product de-signed to eliminate button clicks. Withthe GentleMouse™ users pause (with a configurable time delay) the mousecursor to initiate a click. The delayed pause briefly displays a small,see-through window or trigger window. By moving and pausing once againthe mouse cursor into the trigger window the user can simulate a buttonclick. The GentleMouse™ is being primarily targeted to users withrepetitive strain injuries given that mouse-clicking has been found toaccentuate disorders such as carpal tunnel syndrome [62].

Touchpads are very common input devices on notebooks and provide analternative to mice when working in con-strained spaces. Touchpadimplement the selection with either a physical button or using alift-and-tap technique. MacKenzie and Oniszczak [71] devised afinger-pressing technique with tactile feedback as an alternative toclick and lift-and-tap on a touch pad. In one study, MacKenzie andOniszczak [71] found that with the tactile selection users were 46%faster than with a button click, and 20% compared to the lift-and-tap.

On a stylus, users commonly invoke a selection by directly tapping andthen releasing the stylus over an object. Since tapping does not reflecthow people naturally use notepads, where writing and making checkmarksis common, designers have developed an alternative referred to astouching [80]. Unlike tapping which requires that a pen touch a screenand be lifted directly over the target to select it, touch interactionsonly require that the target be touched at some point. As a result,touching supports selecting targets by crossing them, making checkmarksand even tapping. Results show [68,80] that touching is a viablealternative to tapping for completing selection, even for the elderly[HB]. Other pen-based systems have shown that crossing targets can bemore effective than point and click selections [66]. With CrossY [66]the pointing is eliminated and instead selection happens in one fluidmotion by crossing an object.

Touch screens are also very common and facilitate one of the mostnatural forms of pointing and selecting, by allowing users to selectobjects with a finger. Potter et al [75] compared three selectionmechanisms, take-off, first contact and land-on. Take-off, allows theuser to drag a cursor that appears above the user's finger tip andselect an object by taking off the finger from the touchsceen as thecursor appears in a target. In first-contact the user can drag theirfinger across an empty area of the touchscreen and selects an object bymaking contact with it. Land-on triggers selection the first time thefinger lands on the screen. Their results show that users perform betterwith take-off than with first-contact or land-on [75]. Albins son andZhai [54] ex-tended the work of [75] to design more accurate selectionmechanisms on touchscreens. However their research primarily focused onreducing pointing errors on touchsceens instead of final selectionmechanism.

Pressure-Based Selection Mechanisms

Researchers have proposed using pressure based selection [55,61,71] asan alternative to button-clicking, which we refer to aspressure-clicking. Pressure-clicking has been pro-posed for the mouse[59,83], for touchpads [71], for text-entry [61] and for multi-touchscreens [55].

Several studies discuss the integration of a multi-state pressure buttonto the mouse. Zeleznik et al. [83] proposed an additional “pop-through”state to the mechanical operation of the mouse button. As a result,users can move beyond a simple click or double click by using a numberof techniques that take advantage of a soft-press and a hard-press witha pop-through button, such as shortening/lengthening adaptive menus,character instead of word selection with text, or moving a scroll barwith finer instead of coarser control. Forlines et al. [63] proposed anintermediary “glimpse” state on a mouse-click to facilitate variousediting tasks. Glimpse can be activated using pressure-based selection.With glimpse users can preview the effects of their editing withoutexecuting any commands. Multi-level input can facilitate navigation,editing or selection tasks but utilizes pressure input in a limited way.

On a Synaptics™ touchpad, MacKenzie and Oniszczak [71] facilitatepressure-clicking by giving users aural and haptic feedback on thetouchpad when it is pressed and released. To prevent spurious clicks,the transitions from clicking to releasing (and vice-versa) includehysteresis, i.e. the pressure level that maps to the button-down actionis higher than the pressure-level that maps to the button-up action.However the authors in [71] do not provide the most appropriate pressurelevels to simulate the button clicks and instead suggest that thecorrect thresholds must be determined empirically.

Pressure-clicking has also been employed as an alternative tomulti-tapping buttons on a cell-phone for text-entry [61]. In suchsystems, only a limited number of pressure levels (between 3 to 4) arenecessary to enter text with each key [61]. The authors in [61] presentthe possibility of concurrently combining discrete and continuouspressure input to perform such tasks as zooming or scrolling with largeworkspaces.

Recently, a pseudo pressure-clicking technique, SimPress, wasimplemented for facilitating precise selection techniques for amulti-touch screen [55]. In a non-pressure based input system, Benko etal [55] map changes in the finger's contact area to the changes inpressure. SimPress requires users to perform a small rocking movementwith their finger from the point of contact to the wrist to simulate aclick. With such a mechanism, Benko et al [55] were able to get fairlyaccurate selection rates on a touch-screen.

We designed PButtons to effectively simulate the primary operation ofselection and action invocation. Based on the design framework suggestedby [59] in designing a pressure augmented mouse, the design necessitatescareful consideration to the placement of the sensors, the selectionmechanisms, and the visual feedback In our design we also wantedPButtons to provide a fluid transition from clicking to continuouspressure control.

Selection Techniques with Pressure Buttons

We designed four pressure clicking techniques to operate with pressuresensors: pressure click, pressure click audio, pressure tap, pressuretap audio.

Pressure Click: This selection technique is designed to replicate theoperation of a mouse button click. Applying a pressure Pdown the systeminvoked a mouse down event. Releasing the pressure sensor aftertriggering a mouse down invoked a mouse up when the pressure levelattained a level less than Pup. A pressure-timing graph in FIG. 25depicts the invocation of a mouse down and mouse up with a pressuresensor and a button.

Pressure Click Audio: MacKenzie and Oniszczak [53,71] suggest that auralfeedback is essential to the closed-loop feedback of clicking on a mousebutton. In this selection mode, the system would produce a ‘click’ soundwhen the pressure threshold reached the mouse down and mouse up.

Pressure Tap: In this selection mode a click is registered if the userapplies and releases pressure within a time interval of T. Anythingslower is not registered as a click. Pressure tap was inspired from thecontinuous pressure selection technique referred to as tap-to-switch in[59]. FIG. 26 shows the invocation of a mouse click and release withpressure tap. The entire click-and-release operation is considered asone atomic unit. The click is triggered when the user is capable ofapplying and then releasing a pressure of 2 units within 150 ms. If theuser is not able to apply and release the required pressure within thespecified time interval then the system does not register a click.

Pressure Tap Audio: Pressure tap is missing tactile and aural feedbackand this led to the design of pressure tap audio. mode, which plays amouse down sound when pressure is applied and a mouse up sound if aclick is successfully registered.

Visual Feedback

Based on guidelines from [70,73,78], feedback is a necessary componentfor the proper functioning of pressure input. Unlike mouse buttons,pressure sensors do not provide any aural or tactile feedback upon beingpressed or released. This could adversely affect performance withpressure buttons.

Similar to many other studies, we provide visual feedback with pressurebuttons when the user has invoked a mouse down and mouse up event.However, unlike the outcome of previous results that suggest usingcontinuous visual feed-back (i.e. showing how the user gradually makesit through the pressure space), pressure clicking relies on rapidactions. As a result, PButtons cannot harness any additional benefitsfrom continuous visual feedback. PButtons simply highlights the cursorin orange when the sensor is pressed down and in green when released.

Action Techniques with Pressure Buttons

While selection is a necessary and primary function of mouse buttons,users can also invoke actions such as opening a file or maximizing awindow by double-clicking on a mouse button. Based on the results of thefirst study, PButtons implemented a double-click action registrationmechanism. We designed three action invocation techniques, pressuredouble-click, pressure double-tap and pressure hardpress. Pressuredouble-click triggered a double-click by implementing two pressureclicks followed closely by one another. The time delay between the twopressure clicks is similar to the delay required to register adouble-click using a mouse button. In most systems this delay isconfigurable to match the users motor capacities. Pressure double-taptriggered a double-click by implementing two pressure taps followedclosely by one another. The time delay in between the two pressure tapsis equivalent to the delay assigned to the two pressure clicks in thepressure double-click mechanism. However, to register each pressure tapthe user needs to perform the complete press and release action within150 ms. FIG. 27 depicts the hard-press and sensor double-click in apressure-time graph. Additional specific details on the double-clickmechanisms are provided in the section on double-click mechanisms belowin the paper.

Study of Single-Click Mechanisms

In order to examine the value of pressure sensors in aiding selection wecarried out a study to compare the various techniques for single-click.The main goal of this study was to two-fold, first to see if users cancontrol pressure sensors as buttons without audio feedback and ifpressure-sensor based selection techniques are at least as effective asphysical button-click technique.

Apparatus and Method

Our study used an optical mouse with the pressure sensor mounted ontothe surface of the mouse button (FIG. 24). The sensor (model #IESF-R-5Lfrom CUI Inc.) could measure a maximum pressure value of 1.5 Ns. Eachsensor provided 1024 pressure levels. Pressure sensors are mounted onthe top of each of the two primary mouse buttons. Users could then clickwith the left or right finger to perform a selection. Depending on theinput mode the primary mouse buttons were taped so that they could nolonger be activated by pressing on the sensors. In the condition whenpressure sensors were not tested they were removed from the mouse.

A known limitation of our study is that in comparison to the regularmouse buttons, the pressure buttons covered a very minimal area orfootprint (see FIG. 24). This means that users might make contact withthe pressure sensor with the side of their index or ring finger, whichcould affect the registered system pressure resulting in an error. As aresult users could take longer to potentially trigger a selection withpressure buttons. This we believe is an artefact of our design andprofessionally constructed pressure buttons could alleviate thisimpediment.

The application was developed in C# and the sensor was controlled usingthe Phidgets library [65]. The experiment was conducted in full-screenmode at 1024×768 pixels on a P4 3.0 GHz Windows XP OS.

We used a selection task in which the participants were asked to performa single-click action when a rectangular square turned green. At thebeginning of each trial a timer counts-down to zero, when the squarechanges color from White to Green the user is required to perform thesingle-click action.

We provide a 3-second count-down timer to cue the user so that they areprepared to do the action as quickly as possible when the timer reacheszero. Since we are primarily interested in recording motor-responsetimes we felt this was an effective way to minimize errors.

The user did not have to move the cursor to perform their task and theywere instructed to avoid moving the cursor. However cursor movement wasnot disabled to maintain a task that would be more ecologically valid.During each trial the user performed the selection action using thedifferent selection mechanisms according to the pre-defined order ofpresentation.

Performance Measures

The experimental software recorded trial completion time, and errors asdependent variables. Trial completion time (MT) is defined as the totaltime taken for the user to perform the selection action from the timethe square turned green. The software records an error (E) when theparticipant performed an action but did not complete the selectionaction. For example, in the Pressure-Click mechanism this could occurwhen the user does not press the pressure sensor hard enough for thesystem to register a click. The trial ended only when the user completedthe selection action, so multiple errors were possible for each trial.Participants were also asked in an exit questionnaire to rank thedifferent selection techniques.

Procedure and Design

The study used a 2×5 within-participants factorial design. The factorswere:

Input Device Location: Left side (Index Finger), Right side (MiddleFinger). For the purposes of our study, we are testing the location ofthe primary mouse button, usually controlled with the index finger, andthe location of the secondary mouse button, usually controlled with themiddle finger. These locations are important because they are thelocations of the majority of mouse clicking and mouse button usage.Input Mode Button Click, Pressure Click, Pressure-Click with AudioFeedback, Pressure-Tap, Pressure-Tap with Audio Feedback.

Button click consisted of a single click with the mouse button. InPressure Click (with and without Audio) we use a Pdown of 4 units(sensors collected a range of 1024 discrete pressure units) and a Pup or2 pressure units. In the audio feedback condition, users heard a ‘click’sound when both the Pdown and Pup levels were crossed. For Pressure Tap(with and without Audio) we use a time interval T of 1 50 ms and thesame pressure levels as those used for the pressure click condition. Themajor difference is that users are required to cross both pressurethresholds within the time limit of 150 ms.

The order of presentation was first controlled for input device locationand then for input mode. We explained the input modes and participantswere given ample time to practice the tasks with the various conditionsat the beginning of the experiment. The experiment consisted of threeblocks with each block consisting of twenty repetitions for eachcondition.

Ten participants (4 males and 6 females) between the ages of 19 and 25were recruited from a local university. All subjects had previousexperience with graphical interfaces and used the mouse in their righthand.

With 10 participants, 2 device locations, 5 input modes, 1 task, 3blocks, and 20 trials, the system recorded a total of (10×2×5×1×3×20)6000 trials. The experiment took approximately 40 minutes perparticipant.

Results Completion Time

We used the univariate ANOVA test for our analyses. To make the dataconform to the homogeneity requirements for ANOVA we used a natural logtransform on the completion time and only included in our analysistrials that were successfully completed. However, all the results arepresented on the original untransformed data. Results showed no maineffect of Input Mode and Location on trial completion time withF4,26=2.551 and F1,6=1.198 (p>0.05) respectively. Users were on averagefaster when selecting with the right location. Users were fastest withbutton-click followed by pressure-tap, pressure-tap with audio,pressure-click with audio and pressure-click. FIG. 28 (left) shows themean completion time for each mode grouped by location.

Errors and Subjective Feedback

Across all conditions there were a total of 272 errors. The distributionof errors is as shown in FIG. 28 (right). There were no errors with thebutton-click technique. Seven of the ten subjects preferred button clickwhile three preferred pressure click with audio.

Discussion

We could not detect any difference in completion time between theinteraction modes. Our results show that even though Button-click wasthe fastest selection technique, the difference in selection time withPressure tap was less than 80 ms. As shown in FIG. 29, even though userspress hard with Pressure Tap (peak pressure value ranged from 100 to 300units) users could in one quick action engage and disengage interactionwith the pressure sensor.

As mentioned earlier the footprint of the pressure interactiontechniques was smaller than the footprint of the mouse buttons. Thisdifference did not affect completion times. During the trials usersoften rested their finger on the sensor or the button to reduce deviceacquisition times. But to execute the selection action they had to lifttheir finger and reacquire the sensor. This sometimes resulted in errorsas users sometimes applied pressure at an angle using the side of theirfinger. Where possible the experimenter noted these errors manually andfound that they accounted for 78 of the total number of errors reportedin FIG. 28 (right). If the PButtons are professionally designed webelieve this error-rate would be much lower and could also increase theuser's preference for this style of selection.

Based on our results, we believe that pressure sensors can effectivelyreplace mouse buttons for selection actions.

Study of Double-Click Mechanisms

The results of the first study show that pressure clicking is a viablealternative to mouse clicks. However, the main goal of this study was tosee if users can use pressure-sensors for double-click actions. Theexperimental apparatus and task were similar to those used in theprevious study. The only change is the users is required to perform adouble-click action instead of a single-click action.

The study used a within-participants factorial design with theDouble-click mechanism as the independent variable.

Input Mechanism: Button Click, Pressure Click, Pressure Tap, HardPressand HardPress with audio Feedback. Button click simply consisted of theconventional double-click with the mouse button. Pressure clickconsisted of two consecutive clicks with the pressure sensor. Notime-out delay was used between the two clicks. The pressure value for adown click (Pdown) was 4 units and for a release (Pup) was 2 units.Pressure tap required users to only apply a pressure of P=2 units andrelease within 150 ms, rather than at P=4 units. The HardPress requiredthat each user press beyond a certain activation level, but only once.The major difference between HardPress and the other clicking techniqueswas that the user only needed to press once instead of twice. Since wedid not find any significant difference between the conditions with andwithout audio for Pressure click and Tap in the single-click conditionwe did not include audio-feedback enhanced versions of these techniquesin this study. This also helped us keep the study to a more manageablenumber of independent factors.

Ten participants (5 males and 5 females) between the ages of 19 and 25were recruited from a local university. All subjects had previousexperience with graphical interfaces and used the mouse in their righthand. The order of pres-entation was controlled for input mechanism.

With 10 participants, 5 input modes, 1 task, 3 blocks, and 20 trials,the system recorded a total of (10×5×1×3×20) 3000 trials. The experimenttook approximately 20 minutes per participant.

As with the previous study, the experimental software re-corded trialcompletion time, and errors as dependent variables. Participants werealso asked in an exit questionnaire to rank the different selectiontechniques.

Results Completion Time and Errors

We used the univariate ANOVA test and Tamhane post-hoc pair-wise tests(unequal variances) for all our analyses. To make the data conform tothe homogeneity requirements for ANOVA we used a natural-log transformon the completion time. Results showed main effect of mode (p<0.01) ontrial completion time with F4,29=40. 19.

Post-hoc pair-wise comparisons of Input mode yielded significantdifferences (all p<0.01) in trial completion times for all pairs exceptHardPress, HardPress with Audio and Pressure Click and Pressure Tap.Users were fastest with HardPress followed by HardPress with Audio,Button Press, Pressure Tap and Pressure Click. FIG. 30 (left) shows themean completion time for each mode. There were in total 147 errorsacross all conditions for all trials. The distribution of errors is asshown in FIG. 30 (right). As with the previous study, a large number ofthe errors in the second study resulted from the form factor andfoot-print of the pressure sensors.

Subjective Feedback

In terms of overall preference users were split between the techniquesas shown in FIG. 31 (left) and there was no clear trend. However,approximately 80% of the users preferred some form of pressure clickingmechanism over button click. Surprisingly, none of the subjectspreferred HardPress with Audio.

Discussion

We carried out two independent experiments; one for single-click and theother for double-click. Despite some of the hardware limitations ofPButtons, the results of the first study shows that Pressure click andPressure tap are as good as physical buttons for basic interactions suchas clicking. In the second study we found that users were significantlyfaster when using HardPress for double-click and we found no differencebetween any of the single pressure clicking techniques. Overall theresults of both studies show that PButtons is a potential alternative tomouse clicking and double-clicking. In this section we further discussour observations from the studies. We also present several naturalextensions that can be implemented to make PButtons an accessible toolfor general users and present a brief list of design recommendations todesigners.

Observations on Design of PButtons

Several design elements of PButtons have shown to cause a direct effecton the effectiveness of replacing mouse buttons with pressure sensinginput.

Lack of Continuous Feedback

The feedback component of any closed-loop interaction is crucial to theproper functioning of an interactive system. In the case of buttonclicking, users are given auditory and haptic feedback. Surprisingly,our results did not show any benefits to effects of auditory feedback.This is particularly interesting given that pressure sensing hardwaredoes not provide any accurate form of feedback on its own. Studies havesuggested that at a minimum continuous visual feed-back is necessary forthe successfully operation of pressure based input. However, given thesmall reaction times that were observed, PButtons cannot take fulladvantage of continuous visual feedback to indicate whether theinteraction has arrived at the adequate threshold for triggering apressure click or HardPress. One possible alternative would be toinclude some form of feedback onto the mouse cursor to suggest thenadequate thresholds have been attained.

Double-Click Timeouts

An influencing factor in double click performance is the double-clicktimeout. The timeout that is inherent in double-clicking is importantfor distinguishing a double click from two independent single clicks. Inthe traditional mouse button users can customize and calibrate thetimeout value to optimize their method of operating a double-click. Inthe case of PButtons, Pressure click and Pressure tap techniques alsorely on this timeout to distinguish a double-click from independentsingle-clicks. However, in our second study we deliberately did notimpose any double-click timeouts for any of the interaction techniques.We had an infinite timeout so that we could gauge the average time-outsfor different interaction techniques. FIG. 31 (right) shows the averagedelays for Button click, Pressure Click and Pressure Tap.

Hardpress does not rely on this timeout to distinguish the two. InHardpress, users only need to cross a pressure level to activatedouble-click. However, we noticed that the threshold value for apressure-level varied across users. In our experiments users performedabout 10 practice trials before starting the experimental trails. Weused the values from the practice trials to determine the adequatepressure units that would be necessary to activate a HardPress. In ourstudy users were initiating a HardPress within a thresh-old that rangedfrom 65 to 185 pressure units. We envision that in an actualimplementation of PButtons, users will be able to set the HardPressurethreshold in a manner similar to double-click timeouts as is currentlyperformed in the Windows™ operating system.

PButtons Footprint

Observing users operate with pressure sensors we noticed that a largenumber of errors resulted from improperly positioning the finger on thepressure button. This adversely had an impact on performance as userswould ‘miss’ the ideal pressure spot on the sensor. To alleviate thisproblem, a newer design that considers providing an equal amount of‘footprint’ as that used for regular mouse buttons would improvepressure selection efficiency. This design alternative could alsofacilitate a feedback mechanism similar to mouse buttons.

Natural Extensions to PButtons

The design of PButtons proposed in this paper presents the possibilityof performing a diverse range of operations with a mouse. In thissection we briefly discuss three out of several possible naturalextensions to PButtons that would make this concept a viable alternativeto current methods of selection: pop-through pressure buttons,contextual interaction, and coverage of other basic transactionsaccording to Buxton's three-state model [58]

Pop-Through Pressure Button

The limitations identified by observing the small footprint of thepressure button, led to the idea of possibly installing a sensor underthe mouse button. This variation is very similar to the pop-throughmouse that allows three states [83] but with the additional benefit ofbeing able to access a larger number of modes or pressure values. Thenew pop-through pressure button design would facilitate single clickswith the mouse button. Users can then simulate a double-click using aHardPress mechanism by further applying pressure onto the pop-throughpressure mouse. This pop-through setup would give users the flexibilityto choose pressure interaction or button-based interaction. By pressingfurther onto the mouse, beyond the double-click activation level, theuser can control a large amount of modes in the pressure space. Weinferred that if a pop-through pressure button design is possible, thena large number of interactive features can be associated with a singlepressure sensor.

To identify whether such a concept is even possible in the first place,we tested the pop-through pressure button concept by delegating thesingle click to the mouse button and the double click to the hard press.This assignment of functions (i.e. single click with a mouse button, anddouble click with pressure button) would lead naturally into thepossibility of opening or accessing contextual menus beyond theHardPress activation levels. We were also interested in identifyingwhether performance values with a pop-through setup would be as good asthose obtained in study 1 and study 2. Neither of the two studiesinvestigated the mechanism of moving seamlessly from a discrete to amore continuous mode of selection. We carried out a pilot study toexamine the possibility of going between through multiple forms ofselection.

Results from this initial pilot are positive in suggesting that apop-through pressure button is possible and can facilitate seamlessmovement between different modes of interaction. The mean time forHardPress was approximately 150 ms more than for a button single clickhowever the mean time for a button double click was about 250 ms morethan for button single click. These times are comparable to the averagesreported for study 1 and 2 and we believe it is possible to incorporatethe pressure sensors under the button to facilitate natural progressingfrom discrete to continuous selection and without affecting performance.

From Clicking to Invoking Contextual Pressure Menus

In the second study we did not use an upper pressure threshold forHardPress. Therefore, regardless of how hard users press the sensor, aslong as they crossed the Hard-Press threshold the system activated adouble click. This is useful in many current applications where usersonly use the left mouse button for single or double click actions.However, with pressure sensors users can also activate context menus inother applications like Paint or Word.

For HardPress to serve as a traditional button and to sup-port contextsensitive pressure menus or other similar continuous pressureinteractions HardPress should include an upper pressure threshold. Whenthe user applies a pressure that is beyond a certain pressure value thesystem can then enter into a continuous pressure interaction mode. Thiswould surmount to making the first pressure level correspond to a doubleclick in a multi-level pressure interaction space. In [59] the authorsseveral present techniques that could be adapted to make HardPresseffectively control up to 64 pressure levels. HardPress proposed in thispaper would work in conjunction with the techniques proposed in [59]since the pressure values used in HardPress appear in the lower pressurerange (65-185). This allows designers to user the upper range ofpressure values (>185 pressure units) for continuous pressure-basedinteraction. According to [59] this upper range is sufficient to controla large number of pressure levels.

Extending PButtons to Facilitate Basic Interactions

According to Buxton's three state model interactions with input devicescan be modeled by three basic states: out-of-range (state 1), tracking(state 2) and dragging (state 3). When we consider the state transitionsfor positioning, single click, double click, dragging and clutching, weobserve that, with the exception of dragging, all the other operationsstart at state 1 and return to state 1 (FIG. 32). These can all behandled with the current selection mechanisms. However, draggingnecessitates remaining in state 2 and then returning to state 1 onlywhen the drag operation is completed and the mouse button is released.

To model dragging with pressure sensors, PButtons would need to devisemechanisms to maintain the device in state 2. Although many designs arepossible and would need to be investigated, we propose twoalternatives: 1) Pressure Click&Hold; and 2) PressureLock. With PressureClick&Hold the user would apply pressure beyond the cur-rent activationlevels required for a HardPress. This would result in a switch to state2 which could be maintained for as long as the user maintains pressureon the sensor. However, fine control over pressure levels can bechallenging and PressureLock might be an easier alternative.Pressure-Lock would work similar to ClickLock that is available on mostWindows XP™ based mice. Similar to ClickLock (which can be configured inthe Control Panel in MSWindows™), PressureLock would allow users to dragand drop items without having to keep the pressure sensor held downwhile moving the mouse. Once turned on, the user has to dwell on thepressure sensor for a brief period when selecting an item to move.Afterwards, the user can release the mouse and drag the item. By tappingon the sensor the item would drop to its destination.

Design Recommendations

There are several lessons that designers can take from our experiments:

Pressure buttons can be used as a replacement to mouse buttons tofacilitate discrete and continuous selection mechanisms;

Pressure values in the lower range of the pressure space are adequatefor simulating single or double mouse click functions;

To improve accuracy, the footprint of the pressure buttons needs to beequivalent to that of mouse buttons;

A technique based on the principles of the HardPress should be used tosimulate a double-click mechanism;

Seamless progressing from click, to double-click to controlling a widerange of pressure values is possible with a design similar to thepop-through pressure button.

We investigated the design and evaluation of pressure based interactiontechniques for selection and action invocation. Pressure sensors canpotentially be used instead of the left- and right-click buttons toperform basic single and double-click operations while at the same timeallowing continuous pressure input for more complex applications. Weovercame some of the limitations in pressure sensors associated with thelack of haptic and aural feedback to design a suite of techniques likePressure Click, Pressure Tap and HardPress which we collectively callPButtons. In two user studies that compare PButtons with traditionalmouse buttons we show that PButtons are as good as traditional buttonsfor single click and HardPress is significantly faster than traditionalbutton for double-click.

In this study we only explored the performance of PButtons in terms oftime and accuracy. In future we plan to evaluate the affect sensitivityof PButtons using the Self-Assessment Manikin. We also intend tocarefully examine the design of PButtons that support a more seamlesstransition from clicks to continuous pressure interaction. To do this wewould need to design compelling applications that can leverage all thesefeatures. Some examples of such applications have already been proposedby others [61,78].

Since various modifications can be made in my invention as herein abovedescribed, and many apparently widely different embodiments of same madewithin the spirit and scope of the claims without department from suchspirit and scope, it is intended that all matter contained in theaccompanying specification shall be interpreted as illustrative only andnot in a limiting sense.

REFERENCES

The following references are incorporated herein by reference:

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1. An input device for an electronic device comprising: a housing;electronic circuitry in the housing arranged to detect user inputs andgenerate control signals corresponding to said user inputs to betransmitted to the electronic device; the electronic circuitrycomprising: a first switch arranged to generate a first control signalwhen depressed by the user; a second switch arranged to generate asecond control signal when depressed by the user; at least the secondswitch comprising a pressure sensitive switch arranged to generatecontinuous pressure values in at least two different identifiablediscrete pressure ranges corresponding to different pressures beingapplied by the user in depressing the pressure sensitive switch. 2-8.(canceled)
 9. The input device according to claim 1, for an electronicdevice comprising a plurality of sequential selection items arranged ina plurality of groups, wherein the pressure sensitive switch is arrangedto generate an advancing control signal arranged to advance selection ofone group through the plurality of groups when the pressure sensitiveswitch is momentarily depressed and wherein the discrete pressure rangesare arranged to correspond to the selection items of the selected groupwhen continuous pressure is applied to the pressure sensitive switch forselecting one of the selection items within the selected group.
 10. Theinput device according to claim 9 wherein the electronic circuitrycomprises two pressure sensitive switches, the pressure sensitiveswitches being arranged to generate an advancing control signal arrangedto advance selection of one group through the plurality of groups whenthe pressure sensitive switch is momentarily depressed in opposingdirections relative to one another and wherein the discrete pressureranges of each pressure sensitive switch are arranged to correspond tothe selection items of the selected group when continuous pressure isapplied to the pressure sensitive switch for selecting one of theselection items within the selected group.
 11. (canceled)
 12. (canceled)13. The input device according to claim 1 for use with an electronicdevice including a plurality of sequential selection items to beselected in a range from a first selection item to a last selectionitem, wherein both the first switch and the second switch comprise apressure sensitive switch arranged to generate continuous pressurevalues in at least two different identifiable discrete pressure rangescorresponding to different pressures being applied by the user indepressing the pressure sensitive switch, each discrete pressure rangeof at least one of the pressure sensitive switches corresponding to oneof the selection items whereby increasing pressure applied to said atleast one pressure sensitive switch advances the selection item beingselected towards the last selection item and reducing pressure appliedto said at least one pressure sensitive switch returns the selectionitem being selected towards the first selection item.
 14. The inputdevice according to claim 1 for an electronic device comprising aplurality of sequential selection items arranged in a plurality ofgroups, wherein both the first switch and the second switch comprise apressure sensitive switch arranged to generate continuous pressurevalues in at least two different identifiable discrete pressure rangescorresponding to different pressures being applied by the user indepressing the pressure sensitive switch and wherein at least one of thepressure sensitive switches is arranged to generate an advancing controlsignal arranged to advance selection of one groups through the pluralityof groups when the pressure sensitive switch is momentarily depressedand wherein the discrete pressure ranges of said at least one of thepressure sensitive switches are arranged to correspond to the selectionitems of the selected group when continuous pressure is applied to thepressure sensitive switch for selecting one of the selection itemswithin the selected group.
 15. The input device according to claim 14wherein the two pressure sensitive switches are arranged to generaterespective advancing control signals arranged to advance selection ofone group through the plurality of groups in opposing directionsrelative to one another when the pressure sensitive switches aremomentarily depressed and wherein the discrete pressure ranges of eachpressure sensitive switch are arranged to correspond to the selectionitems of the selected group when continuous pressure is applied to thepressure sensitive switch for selecting one of the selection itemswithin the selected group.
 16. The input device according to claim 1 foruse with an electronic device including a plurality of sequentialselection items to be selected in a range from a first selection item toa last selection item, wherein both the first switch and the secondswitch comprise a pressure sensitive switch arranged to generatecontinuous pressure values in at least two different identifiablediscrete pressure ranges corresponding to different pressures beingapplied by the user in depressing the pressure sensitive switch, eachdiscrete pressure range of at least one of the pressure sensitiveswitches corresponding to one of the selection items whereby increasingpressure applied to said at least one of pressure sensitive switchesadvances the selection item being selected towards the last selectionitem and reducing pressure applied to said at least one of the pressuresensitive switches returns the selection item being selected towards thefirst selection item.
 17. The input device according to claim 1, for anelectronic device comprising a plurality of sequential selection itemsarranged in a plurality of groups, wherein the first switch is arrangedto generate an advancing control signal arranged to advance selection ofone group through the plurality of groups when the first switch ismomentarily depressed and wherein the discrete pressure ranges of thepressure sensitive switch are arranged to correspond to the selectionitems of the selected group when continuous pressure is applied to thepressure sensitive switch for selecting one of the selection itemswithin the selected group.
 18. The input device according to claim 17wherein both the first switch and the second switch comprise pressuresensitive switches.
 19. The input device according to claim 18 whereineach of the two pressure sensitive switches comprises 8 or fewerdiscrete pressure ranges.
 20. The input device according to claim 1 foran electronic device comprising a plurality of sequential selectionitems arranged in cascading levels, wherein both the first switch andthe second switch comprise a pressure sensitive switch arranged togenerate continuous pressure values in at least two differentidentifiable discrete pressure ranges corresponding to differentpressures being applied by the user in depressing, the pressuresensitive switch, the discrete pressure ranges of the pressure sensitiveswitches being arranged to correspond to the selection items ofalternating cascading levels when continuous pressure is applied to thepressure sensitive switch for selecting one of the selection itemswithin a selected level, and wherein switching applied pressure betweenthe two pressure switches is arranged to generate a control signal whichconfirms entry to the electronic device of the selected item within eachlevel.
 21. The input device according to claim 1 wherein both the firstswitch and the second switch comprise a pressure sensitive switcharranged to generate continuous pressure values in at least twodifferent identifiable discrete pressure ranges corresponding todifferent pressures being applied by the user in depressing the pressuresensitive switch and wherein there is provided a third switch comprisinga two state button. 22-25. (canceled)
 26. The input device according toclaim 1 wherein the pressure sensitive switch is arranged to distinguishamong pressure values in discrete pressure ranges corresponding todifferent pressures being applied by the user by dividing the entirerange of pressure values into discrete units with a discretizationfunction and wherein the discretization function is a quadratic functiongiven by the following equation,$y = {{floor}\left( \frac{\left( {x^{2}*l} \right)}{R^{2}} \right)}$where x is the raw pressure value from the pressure switch, I is thenumber of pressure ranges, and R is the total number of raw pressurevalues.
 27. The input device according to claim 1 wherein the pressuresensitive switch is arranged to distinguish among pressure values indiscrete pressure ranges corresponding to different pressures beingapplied by the user by dividing the entire range of pressure values intodiscrete units with a discretization function and wherein thediscretization function arranged to divide the range of pressure valuesinto discrete pressure units such that a currently selected one of thediscrete pressure units is arranged to be larger between respectiveupper and lower pressure value limits than remaining non-selecteddiscrete pressure units.
 28. The input device according to claim 1wherein the pressure sensitive switch is arranged to distinguish amongpressure values in discrete pressure ranges corresponding to differentpressures being applied by the user by dividing the entire range ofpressure values into discrete units with a discretization function andwherein the discretization function is a fisheye function given by thefollowing equation, $y = \left\{ \begin{matrix}{{{floor}\left( \frac{\left( {x - r} \right)*\left( {l - 1} \right)}{{R - r}\;} \right)} + 1} & {x > {r - \frac{R - r}{l - 1}}} \\0 & {x \leq {r - \frac{R - r}{l - 1}}}\end{matrix} \right.$ where x is the raw pressure value from thepressure switch, I is the number of pressure ranges, r is the fisheyeradius, and R is the total number of raw pressure values.
 29. The inputdevice according to claim 1 wherein the electronic circuitry includes atracking mechanism arranged to track movement of the housing relative toa supporting surface and wherein the pressure switch and the trackingmechanism are arranged to controllably vary two different variablefunctions simultaneously.
 30. The input device according to claim 1wherein the pressure switch is operable in a first mode in which avariable function associated with the pressure switch is arranged to bevaried in a first direction responsive to increased pressure applied tothe switch and a second mode in which the variable function is arrangedto be varied in a second direction opposite to the first directionresponsive to increased pressure applied to the switch.
 31. The inputdevice according to claim 1 for an electronic device comprising aselection function and an action initiation function wherein thepressure switch is arranged to generate a first signal responsive to afirst user interaction and a second signal responsive to a second userinteraction, the pressure switch being arranged to generate a selectionsignal responsive to the first and second signals being generated inwhich the selection signal is identifiable as a selection by theselection function of the electronic device. 32-34. (canceled)
 35. Aninput device for an electronic device comprising: a housing; electroniccircuitry in the housing arranged to detect user inputs and generatecontrol signals corresponding to said user inputs to be transmitted tothe electronic device; the electronic circuitry comprising a pressureswitch arranged to generate a control signal when depressed by the usercomprising continuous pressure values in at least two differentidentifiable discrete pressure ranges corresponding to differentpressures being applied by the user in depressing the pressure sensitiveswitch; the pressure sensitive switch being arranged to distinguishamong pressure values in discrete pressure ranges corresponding todifferent pressures being applied by the user by dividing the entirerange of pressure values into discrete units with a discretizationfunction; wherein the discretization function is arranged to divide therange of pressure values into discrete pressure units such that acurrently selected one of the discrete pressure units is arranged to belarger between respective upper and lower pressure value limits thanremaining non-selected discrete pressure units. 36-41. (canceled)
 42. Inan input device for an electronic device comprising a pressure sensitiveswitch arranged to generate continuous pressure values over a range ofpressure values to be transmitted to the electronic device, theimprovement comprising: a discretization function arranged to divide therange of pressure values into discrete pressure units according to thefollowing equation: $y = \left\{ \begin{matrix}{{{floor}\left( \frac{\left( {x - r} \right)*\left( {l - 1} \right)}{R - r} \right)} + 1} & {x > {r - \frac{R - r}{l - 1}}} \\0 & {x \leq {r - \frac{R - r}{l - 1}}}\end{matrix} \right.$ where x is the raw pressure value from thepressure switch, I is the number of pressure ranges, r is the fisheyeradius, and R is the total number of raw pressure values. 43-69.(canceled)